diff --git a/app/config.php b/app/config.php
index 2c5fb252..17d0ffbb 100644
--- a/app/config.php
+++ b/app/config.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 use Colors\Color;
 use function DI\create;
 use function DI\factory;
@@ -241,14 +243,14 @@
             ->withInsertion(new Insertion(Insertion::TYPE_BEFORE, 'ini_set("display_errors", 1);'))
             ->withInsertion(new Insertion(Insertion::TYPE_BEFORE, 'error_reporting(E_ALL);'))
             ->withInsertion(new Insertion(Insertion ::TYPE_BEFORE, 'date_default_timezone_set("Europe/London");'));
-        
+
         return new CodePatcher($c->get(Parser::class), new Standard, $patch);
     },
     FakerGenerator::class => function () {
         return FakerFactory::create();
     },
     RequestRenderer::class => create(),
-    
+
     Terminal::class => factory([TerminalFactory::class, 'fromSystem']),
     'menu' => factory(MenuFactory::class),
     MenuFactory::class => create(),
diff --git a/src/Application.php b/src/Application.php
index 11ef2b29..4fd93ab8 100644
--- a/src/Application.php
+++ b/src/Application.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop;
 
 use DI\ContainerBuilder;
diff --git a/src/Check/CheckInterface.php b/src/Check/CheckInterface.php
index fe435d46..d5db71ad 100644
--- a/src/Check/CheckInterface.php
+++ b/src/Check/CheckInterface.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Check;
 
 /**
diff --git a/src/Check/CheckRepository.php b/src/Check/CheckRepository.php
index 19f245d3..87ee2364 100644
--- a/src/Check/CheckRepository.php
+++ b/src/Check/CheckRepository.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Check;
 
 use PhpSchool\PhpWorkshop\Exception\InvalidArgumentException;
diff --git a/src/Check/CodeParseCheck.php b/src/Check/CodeParseCheck.php
index 69368914..b697889f 100644
--- a/src/Check/CodeParseCheck.php
+++ b/src/Check/CodeParseCheck.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Check;
 
 use PhpParser\Error;
diff --git a/src/Check/ComposerCheck.php b/src/Check/ComposerCheck.php
index 73d4691c..3b663853 100644
--- a/src/Check/ComposerCheck.php
+++ b/src/Check/ComposerCheck.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Check;
 
 use PhpSchool\PhpWorkshop\ComposerUtil\LockFileParser;
diff --git a/src/Check/DatabaseCheck.php b/src/Check/DatabaseCheck.php
index d7e18ae6..29de3f68 100644
--- a/src/Check/DatabaseCheck.php
+++ b/src/Check/DatabaseCheck.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Check;
 
 use PDO;
diff --git a/src/Check/FileExistsCheck.php b/src/Check/FileExistsCheck.php
index 8327ff94..ff0952b9 100644
--- a/src/Check/FileExistsCheck.php
+++ b/src/Check/FileExistsCheck.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Check;
 
 use PhpSchool\PhpWorkshop\Exercise\ExerciseInterface;
diff --git a/src/Check/FunctionRequirementsCheck.php b/src/Check/FunctionRequirementsCheck.php
index 9446a5a2..2684b5d9 100644
--- a/src/Check/FunctionRequirementsCheck.php
+++ b/src/Check/FunctionRequirementsCheck.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Check;
 
 use PhpParser\Error;
diff --git a/src/Check/ListenableCheckInterface.php b/src/Check/ListenableCheckInterface.php
index a7e705d0..8995b72d 100644
--- a/src/Check/ListenableCheckInterface.php
+++ b/src/Check/ListenableCheckInterface.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Check;
 
 use PhpSchool\PhpWorkshop\Event\EventDispatcher;
diff --git a/src/Check/PhpLintCheck.php b/src/Check/PhpLintCheck.php
index e6cc6ed3..9d148251 100644
--- a/src/Check/PhpLintCheck.php
+++ b/src/Check/PhpLintCheck.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Check;
 
 use PhpSchool\PhpWorkshop\Exercise\ExerciseInterface;
diff --git a/src/Check/SimpleCheckInterface.php b/src/Check/SimpleCheckInterface.php
index 44e62332..af72aa9a 100644
--- a/src/Check/SimpleCheckInterface.php
+++ b/src/Check/SimpleCheckInterface.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Check;
 
 use PhpSchool\PhpWorkshop\Exercise\ExerciseInterface;
diff --git a/src/CodeInsertion.php b/src/CodeInsertion.php
index d7cd8fa1..981fa79b 100644
--- a/src/CodeInsertion.php
+++ b/src/CodeInsertion.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop;
 
 use PhpSchool\PhpWorkshop\Exception\InvalidArgumentException;
diff --git a/src/CodePatcher.php b/src/CodePatcher.php
index 80274c6d..c6ba019a 100644
--- a/src/CodePatcher.php
+++ b/src/CodePatcher.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop;
 
 use PhpParser\Error;
diff --git a/src/Command/CreditsCommand.php b/src/Command/CreditsCommand.php
index 3f37656d..6a7f7f42 100644
--- a/src/Command/CreditsCommand.php
+++ b/src/Command/CreditsCommand.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Command;
 
 use Colors\Color;
diff --git a/src/Command/HelpCommand.php b/src/Command/HelpCommand.php
index 11756c37..5c73babd 100644
--- a/src/Command/HelpCommand.php
+++ b/src/Command/HelpCommand.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Command;
 
 use Colors\Color;
@@ -43,7 +45,7 @@ public function __construct(string $appName, OutputInterface $output, Color $col
      */
     public function __invoke(): void
     {
-        $this->output->writeLine($this->color->__invoke('Usage')->yellow()->bold());
+        $this->output->writeLine((string) $this->color->__invoke('Usage')->yellow()->bold());
         $this->output->writeLine("");
         $this->output->writeLine(sprintf("  %s", $this->color->__invoke($this->appName)->green()));
         $this->output->writeLine("    Show a menu to interactively select a workshop.");
@@ -57,7 +59,7 @@ public function __invoke(): void
         $this->output->writeLine("    Show the people who made this happen.");
         $this->output->writeLine("");
         $this->output->writeLine(
-            $this->color->__invoke('Having trouble with a PHPSchool exercise?')->yellow()->bold()
+            (string) $this->color->__invoke('Having trouble with a PHPSchool exercise?')->yellow()->bold()
         );
         $this->output->writeLine("");
         $this->output->writeLine("  A team of expert helper elves is eagerly waiting to assist you in");
@@ -72,7 +74,7 @@ public function __invoke(): void
         $this->output->writeLine("    https://chat.stackoverflow.com/rooms/11/php");
         $this->output->writeLine("");
         $this->output->writeLine(
-            $this->color->__invoke('Found a bug with PHPSchool or just want to contribute?')->yellow()->bold()
+            (string) $this->color->__invoke('Found a bug with PHPSchool or just want to contribute?')->yellow()->bold()
         );
         $this->output->writeLine("  The official repository for PHPSchool is:");
         $this->output->writeLine("    https://github.com/php-school/php-workshop");
diff --git a/src/Command/MenuCommand.php b/src/Command/MenuCommand.php
index b925c912..58b58394 100644
--- a/src/Command/MenuCommand.php
+++ b/src/Command/MenuCommand.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Command;
 
 use PhpSchool\CliMenu\CliMenu;
diff --git a/src/Command/MenuCommandInvoker.php b/src/Command/MenuCommandInvoker.php
index 2faaea33..b0cf841a 100644
--- a/src/Command/MenuCommandInvoker.php
+++ b/src/Command/MenuCommandInvoker.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Command;
 
 use PhpSchool\CliMenu\CliMenu;
diff --git a/src/Command/PrintCommand.php b/src/Command/PrintCommand.php
index 9850e3f1..7e63bb58 100644
--- a/src/Command/PrintCommand.php
+++ b/src/Command/PrintCommand.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Command;
 
 use PhpSchool\PhpWorkshop\Exercise\ExerciseInterface;
diff --git a/src/Command/RunCommand.php b/src/Command/RunCommand.php
index ccbfc7e3..71c68b2f 100644
--- a/src/Command/RunCommand.php
+++ b/src/Command/RunCommand.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Command;
 
 use PhpSchool\PhpWorkshop\ExerciseDispatcher;
diff --git a/src/Command/VerifyCommand.php b/src/Command/VerifyCommand.php
index d939c210..416c0799 100644
--- a/src/Command/VerifyCommand.php
+++ b/src/Command/VerifyCommand.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Command;
 
 use PhpSchool\PhpWorkshop\ExerciseDispatcher;
diff --git a/src/CommandArgument.php b/src/CommandArgument.php
index 3db84e1c..fcb49b93 100644
--- a/src/CommandArgument.php
+++ b/src/CommandArgument.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop;
 
 /**
diff --git a/src/CommandDefinition.php b/src/CommandDefinition.php
index b46b07e5..e8996aaf 100644
--- a/src/CommandDefinition.php
+++ b/src/CommandDefinition.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop;
 
 use PhpSchool\PhpWorkshop\Exception\InvalidArgumentException;
diff --git a/src/CommandRouter.php b/src/CommandRouter.php
index 469c6db8..c86a3b20 100644
--- a/src/CommandRouter.php
+++ b/src/CommandRouter.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop;
 
 use PhpSchool\PhpWorkshop\Event\EventDispatcher;
diff --git a/src/ComposerUtil/LockFileParser.php b/src/ComposerUtil/LockFileParser.php
index f89ef82d..d6b14d26 100644
--- a/src/ComposerUtil/LockFileParser.php
+++ b/src/ComposerUtil/LockFileParser.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\ComposerUtil;
 
 use PhpSchool\PhpWorkshop\Exception\InvalidArgumentException;
diff --git a/src/Event/CgiExecuteEvent.php b/src/Event/CgiExecuteEvent.php
index e174aa14..e72551b6 100644
--- a/src/Event/CgiExecuteEvent.php
+++ b/src/Event/CgiExecuteEvent.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Event;
 
 use Psr\Http\Message\RequestInterface;
diff --git a/src/Event/CliExecuteEvent.php b/src/Event/CliExecuteEvent.php
index ea2e5361..42cb5581 100644
--- a/src/Event/CliExecuteEvent.php
+++ b/src/Event/CliExecuteEvent.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Event;
 
 use PhpSchool\PhpWorkshop\Utils\ArrayObject;
diff --git a/src/Event/ContainerListenerHelper.php b/src/Event/ContainerListenerHelper.php
index 35a2594b..4bd8f323 100644
--- a/src/Event/ContainerListenerHelper.php
+++ b/src/Event/ContainerListenerHelper.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Event;
 
 /**
diff --git a/src/Event/Event.php b/src/Event/Event.php
index 9abe8d26..f01aa4aa 100644
--- a/src/Event/Event.php
+++ b/src/Event/Event.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Event;
 
 use PhpSchool\PhpWorkshop\Exception\InvalidArgumentException;
diff --git a/src/Event/EventDispatcher.php b/src/Event/EventDispatcher.php
index b8c7ee5e..dfacca14 100644
--- a/src/Event/EventDispatcher.php
+++ b/src/Event/EventDispatcher.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Event;
 
 use PhpSchool\PhpWorkshop\Result\ResultInterface;
diff --git a/src/Event/EventInterface.php b/src/Event/EventInterface.php
index 2698989f..e586d93d 100644
--- a/src/Event/EventInterface.php
+++ b/src/Event/EventInterface.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Event;
 
 use PhpSchool\PhpWorkshop\Exception\InvalidArgumentException;
diff --git a/src/Event/ExerciseRunnerEvent.php b/src/Event/ExerciseRunnerEvent.php
index 06eedcd8..0dc9ce4a 100644
--- a/src/Event/ExerciseRunnerEvent.php
+++ b/src/Event/ExerciseRunnerEvent.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Event;
 
 use PhpSchool\PhpWorkshop\Exercise\ExerciseInterface;
diff --git a/src/Event/functions.php b/src/Event/functions.php
index 62b8bfbe..818d470e 100644
--- a/src/Event/functions.php
+++ b/src/Event/functions.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Event;
 
 if (!function_exists('PhpSchool\PhpWorkshop\Event\containerListener')) {
diff --git a/src/Exception/CheckNotApplicableException.php b/src/Exception/CheckNotApplicableException.php
index 0f93fe51..68d7f0bd 100644
--- a/src/Exception/CheckNotApplicableException.php
+++ b/src/Exception/CheckNotApplicableException.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Exception;
 
 use PhpSchool\PhpWorkshop\Check\CheckInterface;
diff --git a/src/Exception/CliRouteNotExistsException.php b/src/Exception/CliRouteNotExistsException.php
index 494293cd..aadc3801 100644
--- a/src/Exception/CliRouteNotExistsException.php
+++ b/src/Exception/CliRouteNotExistsException.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Exception;
 
 use RuntimeException;
diff --git a/src/Exception/CodeExecutionException.php b/src/Exception/CodeExecutionException.php
index ea6cc45b..2e4e5561 100644
--- a/src/Exception/CodeExecutionException.php
+++ b/src/Exception/CodeExecutionException.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Exception;
 
 use RuntimeException;
diff --git a/src/Exception/ExerciseNotConfiguredException.php b/src/Exception/ExerciseNotConfiguredException.php
index f3163d03..81b3cafb 100644
--- a/src/Exception/ExerciseNotConfiguredException.php
+++ b/src/Exception/ExerciseNotConfiguredException.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Exception;
 
 use PhpSchool\PhpWorkshop\Exercise\ExerciseInterface;
diff --git a/src/Exception/InvalidArgumentException.php b/src/Exception/InvalidArgumentException.php
index 3db70f61..038f389a 100644
--- a/src/Exception/InvalidArgumentException.php
+++ b/src/Exception/InvalidArgumentException.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Exception;
 
 /**
diff --git a/src/Exception/MissingArgumentException.php b/src/Exception/MissingArgumentException.php
index 3b4937c5..552d5e72 100644
--- a/src/Exception/MissingArgumentException.php
+++ b/src/Exception/MissingArgumentException.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Exception;
 
 use RuntimeException;
diff --git a/src/Exception/SolutionExecutionException.php b/src/Exception/SolutionExecutionException.php
index 2bf5d88b..1393ee9a 100644
--- a/src/Exception/SolutionExecutionException.php
+++ b/src/Exception/SolutionExecutionException.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Exception;
 
 use RuntimeException;
diff --git a/src/Exercise/AbstractExercise.php b/src/Exercise/AbstractExercise.php
index 5488aa87..b94d204d 100644
--- a/src/Exercise/AbstractExercise.php
+++ b/src/Exercise/AbstractExercise.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Exercise;
 
 use PhpSchool\PhpWorkshop\ExerciseDispatcher;
diff --git a/src/Exercise/CgiExercise.php b/src/Exercise/CgiExercise.php
index 0cbe1152..1244712a 100644
--- a/src/Exercise/CgiExercise.php
+++ b/src/Exercise/CgiExercise.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Exercise;
 
 use Psr\Http\Message\RequestInterface;
diff --git a/src/Exercise/CliExercise.php b/src/Exercise/CliExercise.php
index b7edf9da..80a287f9 100644
--- a/src/Exercise/CliExercise.php
+++ b/src/Exercise/CliExercise.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Exercise;
 
 /**
diff --git a/src/Exercise/CustomVerifyingExercise.php b/src/Exercise/CustomVerifyingExercise.php
index bd5580db..ab8c2adf 100644
--- a/src/Exercise/CustomVerifyingExercise.php
+++ b/src/Exercise/CustomVerifyingExercise.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Exercise;
 
 use PhpSchool\PhpWorkshop\Result\ResultInterface;
diff --git a/src/Exercise/ExerciseInterface.php b/src/Exercise/ExerciseInterface.php
index ff16ba64..fb3e8fc8 100644
--- a/src/Exercise/ExerciseInterface.php
+++ b/src/Exercise/ExerciseInterface.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Exercise;
 
 use PhpSchool\PhpWorkshop\ExerciseDispatcher;
diff --git a/src/Exercise/ExerciseType.php b/src/Exercise/ExerciseType.php
index 4259947b..7baabc6f 100644
--- a/src/Exercise/ExerciseType.php
+++ b/src/Exercise/ExerciseType.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Exercise;
 
 use MyCLabs\Enum\Enum;
diff --git a/src/Exercise/ProvidesSolution.php b/src/Exercise/ProvidesSolution.php
index ee325076..bffc74d9 100644
--- a/src/Exercise/ProvidesSolution.php
+++ b/src/Exercise/ProvidesSolution.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Exercise;
 
 use PhpSchool\PhpWorkshop\Solution\SolutionInterface;
diff --git a/src/Exercise/SubmissionPatchable.php b/src/Exercise/SubmissionPatchable.php
index de3ce70b..3655ac42 100644
--- a/src/Exercise/SubmissionPatchable.php
+++ b/src/Exercise/SubmissionPatchable.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Exercise;
 
 use PhpSchool\PhpWorkshop\Patch;
diff --git a/src/Exercise/TemporaryDirectoryTrait.php b/src/Exercise/TemporaryDirectoryTrait.php
index d8e11e8c..49a450a4 100644
--- a/src/Exercise/TemporaryDirectoryTrait.php
+++ b/src/Exercise/TemporaryDirectoryTrait.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Exercise;
 
 /**
diff --git a/src/ExerciseCheck/ComposerExerciseCheck.php b/src/ExerciseCheck/ComposerExerciseCheck.php
index 568863fe..d72e89c4 100644
--- a/src/ExerciseCheck/ComposerExerciseCheck.php
+++ b/src/ExerciseCheck/ComposerExerciseCheck.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\ExerciseCheck;
 
 /**
diff --git a/src/ExerciseCheck/DatabaseExerciseCheck.php b/src/ExerciseCheck/DatabaseExerciseCheck.php
index 1c99de88..7e176491 100644
--- a/src/ExerciseCheck/DatabaseExerciseCheck.php
+++ b/src/ExerciseCheck/DatabaseExerciseCheck.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\ExerciseCheck;
 
 use PDO;
diff --git a/src/ExerciseCheck/FunctionRequirementsExerciseCheck.php b/src/ExerciseCheck/FunctionRequirementsExerciseCheck.php
index 85570f52..8641ec9a 100644
--- a/src/ExerciseCheck/FunctionRequirementsExerciseCheck.php
+++ b/src/ExerciseCheck/FunctionRequirementsExerciseCheck.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\ExerciseCheck;
 
 /**
diff --git a/src/ExerciseCheck/SelfCheck.php b/src/ExerciseCheck/SelfCheck.php
index a98bb08e..857cf850 100644
--- a/src/ExerciseCheck/SelfCheck.php
+++ b/src/ExerciseCheck/SelfCheck.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\ExerciseCheck;
 
 use PhpSchool\PhpWorkshop\Input\Input;
diff --git a/src/ExerciseDispatcher.php b/src/ExerciseDispatcher.php
index b4dfc05f..77230edc 100644
--- a/src/ExerciseDispatcher.php
+++ b/src/ExerciseDispatcher.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop;
 
 use PhpSchool\PhpWorkshop\Check\CheckInterface;
diff --git a/src/ExerciseRenderer.php b/src/ExerciseRenderer.php
index 20690d27..f578945b 100644
--- a/src/ExerciseRenderer.php
+++ b/src/ExerciseRenderer.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop;
 
 use Colors\Color;
diff --git a/src/ExerciseRepository.php b/src/ExerciseRepository.php
index 56fabf74..5314999a 100644
--- a/src/ExerciseRepository.php
+++ b/src/ExerciseRepository.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop;
 
 use ArrayIterator;
diff --git a/src/ExerciseRunner/CgiRunner.php b/src/ExerciseRunner/CgiRunner.php
index c18ad2ae..9a8eb9f4 100644
--- a/src/ExerciseRunner/CgiRunner.php
+++ b/src/ExerciseRunner/CgiRunner.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\ExerciseRunner;
 
 use GuzzleHttp\Psr7\Message;
diff --git a/src/ExerciseRunner/CliRunner.php b/src/ExerciseRunner/CliRunner.php
index bf5728e9..3aca8dc0 100644
--- a/src/ExerciseRunner/CliRunner.php
+++ b/src/ExerciseRunner/CliRunner.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\ExerciseRunner;
 
 use PhpSchool\PhpWorkshop\Check\CodeParseCheck;
diff --git a/src/ExerciseRunner/CustomVerifyingRunner.php b/src/ExerciseRunner/CustomVerifyingRunner.php
index 65297896..c2fb9afc 100644
--- a/src/ExerciseRunner/CustomVerifyingRunner.php
+++ b/src/ExerciseRunner/CustomVerifyingRunner.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\ExerciseRunner;
 
 use PhpSchool\PhpWorkshop\Exercise\CustomVerifyingExercise;
diff --git a/src/ExerciseRunner/ExerciseRunnerInterface.php b/src/ExerciseRunner/ExerciseRunnerInterface.php
index 8a94af4f..42dbb023 100644
--- a/src/ExerciseRunner/ExerciseRunnerInterface.php
+++ b/src/ExerciseRunner/ExerciseRunnerInterface.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\ExerciseRunner;
 
 use PhpSchool\PhpWorkshop\Exercise\ExerciseInterface;
diff --git a/src/ExerciseRunner/Factory/CgiRunnerFactory.php b/src/ExerciseRunner/Factory/CgiRunnerFactory.php
index 7b574895..75e9d38b 100644
--- a/src/ExerciseRunner/Factory/CgiRunnerFactory.php
+++ b/src/ExerciseRunner/Factory/CgiRunnerFactory.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\ExerciseRunner\Factory;
 
 use PhpSchool\PhpWorkshop\CommandArgument;
diff --git a/src/ExerciseRunner/Factory/CliRunnerFactory.php b/src/ExerciseRunner/Factory/CliRunnerFactory.php
index 0461afdf..9694d642 100644
--- a/src/ExerciseRunner/Factory/CliRunnerFactory.php
+++ b/src/ExerciseRunner/Factory/CliRunnerFactory.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\ExerciseRunner\Factory;
 
 use PhpSchool\PhpWorkshop\CommandArgument;
diff --git a/src/ExerciseRunner/Factory/CustomVerifyingRunnerFactory.php b/src/ExerciseRunner/Factory/CustomVerifyingRunnerFactory.php
index b3d5de18..6ef11290 100644
--- a/src/ExerciseRunner/Factory/CustomVerifyingRunnerFactory.php
+++ b/src/ExerciseRunner/Factory/CustomVerifyingRunnerFactory.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\ExerciseRunner\Factory;
 
 use PhpSchool\PhpWorkshop\CommandDefinition;
diff --git a/src/ExerciseRunner/Factory/ExerciseRunnerFactoryInterface.php b/src/ExerciseRunner/Factory/ExerciseRunnerFactoryInterface.php
index 683023d0..e36f9a31 100644
--- a/src/ExerciseRunner/Factory/ExerciseRunnerFactoryInterface.php
+++ b/src/ExerciseRunner/Factory/ExerciseRunnerFactoryInterface.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\ExerciseRunner\Factory;
 
 use PhpSchool\PhpWorkshop\CommandDefinition;
diff --git a/src/ExerciseRunner/RunnerManager.php b/src/ExerciseRunner/RunnerManager.php
index 1dbaf025..149022e0 100644
--- a/src/ExerciseRunner/RunnerManager.php
+++ b/src/ExerciseRunner/RunnerManager.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\ExerciseRunner;
 
 use PhpSchool\PhpWorkshop\CommandDefinition;
diff --git a/src/Factory/EventDispatcherFactory.php b/src/Factory/EventDispatcherFactory.php
index 16d0303c..f6c94888 100644
--- a/src/Factory/EventDispatcherFactory.php
+++ b/src/Factory/EventDispatcherFactory.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Factory;
 
 use Psr\Container\ContainerInterface;
diff --git a/src/Factory/MarkdownCliRendererFactory.php b/src/Factory/MarkdownCliRendererFactory.php
index 7a3e8b32..4275a88f 100644
--- a/src/Factory/MarkdownCliRendererFactory.php
+++ b/src/Factory/MarkdownCliRendererFactory.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Factory;
 
 use AydinHassan\CliMdRenderer\Renderer\ListBlockRenderer;
diff --git a/src/Factory/MenuFactory.php b/src/Factory/MenuFactory.php
index 91fd1a6e..3249018d 100644
--- a/src/Factory/MenuFactory.php
+++ b/src/Factory/MenuFactory.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Factory;
 
 use PhpSchool\CliMenu\Style\SelectableStyle;
diff --git a/src/Factory/ResultRendererFactory.php b/src/Factory/ResultRendererFactory.php
index fb9f89ac..10109a90 100644
--- a/src/Factory/ResultRendererFactory.php
+++ b/src/Factory/ResultRendererFactory.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Factory;
 
 use PhpSchool\PhpWorkshop\Exception\InvalidArgumentException;
diff --git a/src/Input/Input.php b/src/Input/Input.php
index 192534f8..4d0a0d81 100644
--- a/src/Input/Input.php
+++ b/src/Input/Input.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Input;
 
 use PhpSchool\PhpWorkshop\CommandDefinition;
diff --git a/src/Listener/CheckExerciseAssignedListener.php b/src/Listener/CheckExerciseAssignedListener.php
index d7750ddb..4389dcd4 100644
--- a/src/Listener/CheckExerciseAssignedListener.php
+++ b/src/Listener/CheckExerciseAssignedListener.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Listener;
 
 use PhpSchool\PhpWorkshop\CommandDefinition;
diff --git a/src/Listener/CodePatchListener.php b/src/Listener/CodePatchListener.php
index b38e7d5c..26d44ed4 100644
--- a/src/Listener/CodePatchListener.php
+++ b/src/Listener/CodePatchListener.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Listener;
 
 use PhpSchool\PhpWorkshop\CodePatcher;
diff --git a/src/Listener/ConfigureCommandListener.php b/src/Listener/ConfigureCommandListener.php
index cc032bf5..c519ef82 100644
--- a/src/Listener/ConfigureCommandListener.php
+++ b/src/Listener/ConfigureCommandListener.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Listener;
 
 use PhpSchool\PhpWorkshop\CommandDefinition;
diff --git a/src/Listener/PrepareSolutionListener.php b/src/Listener/PrepareSolutionListener.php
index 16c77f1e..5aa46834 100644
--- a/src/Listener/PrepareSolutionListener.php
+++ b/src/Listener/PrepareSolutionListener.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Listener;
 
 use PhpSchool\PhpWorkshop\Event\ExerciseRunnerEvent;
diff --git a/src/Listener/RealPathListener.php b/src/Listener/RealPathListener.php
index 2d8b8207..6a8be51a 100644
--- a/src/Listener/RealPathListener.php
+++ b/src/Listener/RealPathListener.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Listener;
 
 use PhpSchool\PhpWorkshop\CommandDefinition;
diff --git a/src/Listener/SelfCheckListener.php b/src/Listener/SelfCheckListener.php
index e0773a92..424e5675 100644
--- a/src/Listener/SelfCheckListener.php
+++ b/src/Listener/SelfCheckListener.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Listener;
 
 use PhpSchool\PhpWorkshop\Event\Event;
diff --git a/src/MarkdownRenderer.php b/src/MarkdownRenderer.php
index 2529cde3..30390578 100644
--- a/src/MarkdownRenderer.php
+++ b/src/MarkdownRenderer.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop;
 
 use League\CommonMark\DocParser;
diff --git a/src/MenuItem/ResetProgress.php b/src/MenuItem/ResetProgress.php
index 20399f5a..67d654c7 100644
--- a/src/MenuItem/ResetProgress.php
+++ b/src/MenuItem/ResetProgress.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\MenuItem;
 
 use PhpSchool\CliMenu\CliMenu;
diff --git a/src/NodeVisitor/FunctionVisitor.php b/src/NodeVisitor/FunctionVisitor.php
index 5661b3c6..e2c5d03b 100644
--- a/src/NodeVisitor/FunctionVisitor.php
+++ b/src/NodeVisitor/FunctionVisitor.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\NodeVisitor;
 
 use PhpParser\Node;
diff --git a/src/Output/OutputInterface.php b/src/Output/OutputInterface.php
index dbbc6d2d..5bcf1433 100644
--- a/src/Output/OutputInterface.php
+++ b/src/Output/OutputInterface.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Output;
 
 use Psr\Http\Message\RequestInterface;
diff --git a/src/Output/StdOutput.php b/src/Output/StdOutput.php
index 95260f5b..f2d61f12 100644
--- a/src/Output/StdOutput.php
+++ b/src/Output/StdOutput.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Output;
 
 use Colors\Color;
diff --git a/src/Patch.php b/src/Patch.php
index b1083459..aabb2697 100644
--- a/src/Patch.php
+++ b/src/Patch.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop;
 
 use Closure;
diff --git a/src/Result/Cgi/CgiResult.php b/src/Result/Cgi/CgiResult.php
index 8b1e0446..151ff7f7 100644
--- a/src/Result/Cgi/CgiResult.php
+++ b/src/Result/Cgi/CgiResult.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Result\Cgi;
 
 use ArrayIterator;
diff --git a/src/Result/Cgi/FailureInterface.php b/src/Result/Cgi/FailureInterface.php
index 76d57644..ef782274 100644
--- a/src/Result/Cgi/FailureInterface.php
+++ b/src/Result/Cgi/FailureInterface.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Result\Cgi;
 
 /**
diff --git a/src/Result/Cgi/GenericFailure.php b/src/Result/Cgi/GenericFailure.php
index 52cf42fb..75d4c8a4 100644
--- a/src/Result/Cgi/GenericFailure.php
+++ b/src/Result/Cgi/GenericFailure.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Result\Cgi;
 
 use PhpSchool\PhpWorkshop\Exception\CodeExecutionException;
diff --git a/src/Result/Cgi/RequestFailure.php b/src/Result/Cgi/RequestFailure.php
index 65c4f1ce..0caa8442 100644
--- a/src/Result/Cgi/RequestFailure.php
+++ b/src/Result/Cgi/RequestFailure.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Result\Cgi;
 
 use Psr\Http\Message\RequestInterface;
diff --git a/src/Result/Cgi/ResultInterface.php b/src/Result/Cgi/ResultInterface.php
index e54424ab..3b20c59d 100644
--- a/src/Result/Cgi/ResultInterface.php
+++ b/src/Result/Cgi/ResultInterface.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Result\Cgi;
 
 use Psr\Http\Message\RequestInterface;
diff --git a/src/Result/Cgi/Success.php b/src/Result/Cgi/Success.php
index 25f7873b..df92a327 100644
--- a/src/Result/Cgi/Success.php
+++ b/src/Result/Cgi/Success.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Result\Cgi;
 
 use PhpSchool\PhpWorkshop\Check\CheckInterface;
diff --git a/src/Result/Cgi/SuccessInterface.php b/src/Result/Cgi/SuccessInterface.php
index 1ac42250..96d89a3a 100644
--- a/src/Result/Cgi/SuccessInterface.php
+++ b/src/Result/Cgi/SuccessInterface.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Result\Cgi;
 
 /**
diff --git a/src/Result/Cli/CliResult.php b/src/Result/Cli/CliResult.php
index 02082588..178a24ca 100644
--- a/src/Result/Cli/CliResult.php
+++ b/src/Result/Cli/CliResult.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Result\Cli;
 
 use ArrayIterator;
diff --git a/src/Result/Cli/FailureInterface.php b/src/Result/Cli/FailureInterface.php
index 4e48ff73..429f7c21 100644
--- a/src/Result/Cli/FailureInterface.php
+++ b/src/Result/Cli/FailureInterface.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Result\Cli;
 
 /**
diff --git a/src/Result/Cli/GenericFailure.php b/src/Result/Cli/GenericFailure.php
index 4ec12974..63504866 100644
--- a/src/Result/Cli/GenericFailure.php
+++ b/src/Result/Cli/GenericFailure.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Result\Cli;
 
 use PhpSchool\PhpWorkshop\Exception\CodeExecutionException;
diff --git a/src/Result/Cli/RequestFailure.php b/src/Result/Cli/RequestFailure.php
index 312ca321..f264fd14 100644
--- a/src/Result/Cli/RequestFailure.php
+++ b/src/Result/Cli/RequestFailure.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Result\Cli;
 
 use PhpSchool\PhpWorkshop\Utils\ArrayObject;
diff --git a/src/Result/Cli/ResultInterface.php b/src/Result/Cli/ResultInterface.php
index 08135cfc..319d6a93 100644
--- a/src/Result/Cli/ResultInterface.php
+++ b/src/Result/Cli/ResultInterface.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Result\Cli;
 
 use PhpSchool\PhpWorkshop\Utils\ArrayObject;
diff --git a/src/Result/Cli/Success.php b/src/Result/Cli/Success.php
index 9ab24e26..05227669 100644
--- a/src/Result/Cli/Success.php
+++ b/src/Result/Cli/Success.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Result\Cli;
 
 use PhpSchool\PhpWorkshop\Utils\ArrayObject;
diff --git a/src/Result/Cli/SuccessInterface.php b/src/Result/Cli/SuccessInterface.php
index 9572fc6e..bf4b4717 100644
--- a/src/Result/Cli/SuccessInterface.php
+++ b/src/Result/Cli/SuccessInterface.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Result\Cli;
 
 /**
diff --git a/src/Result/ComparisonFailure.php b/src/Result/ComparisonFailure.php
index 2d28c073..7ecde542 100644
--- a/src/Result/ComparisonFailure.php
+++ b/src/Result/ComparisonFailure.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Result;
 
 /**
diff --git a/src/Result/Failure.php b/src/Result/Failure.php
index a00c9d5a..31d91cc5 100644
--- a/src/Result/Failure.php
+++ b/src/Result/Failure.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Result;
 
 use PhpParser\Error as ParseErrorException;
diff --git a/src/Result/FailureInterface.php b/src/Result/FailureInterface.php
index 3a1d5e4b..468485f7 100644
--- a/src/Result/FailureInterface.php
+++ b/src/Result/FailureInterface.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Result;
 
 /**
diff --git a/src/Result/FunctionRequirementsFailure.php b/src/Result/FunctionRequirementsFailure.php
index 80a5d0b9..3c69f1fc 100644
--- a/src/Result/FunctionRequirementsFailure.php
+++ b/src/Result/FunctionRequirementsFailure.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Result;
 
 use PhpSchool\PhpWorkshop\Check\CheckInterface;
diff --git a/src/Result/ResultGroupInterface.php b/src/Result/ResultGroupInterface.php
index d2d23cbe..e318ee63 100644
--- a/src/Result/ResultGroupInterface.php
+++ b/src/Result/ResultGroupInterface.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Result;
 
 /**
diff --git a/src/Result/ResultInterface.php b/src/Result/ResultInterface.php
index c4fa891a..ad46cc94 100644
--- a/src/Result/ResultInterface.php
+++ b/src/Result/ResultInterface.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Result;
 
 /**
diff --git a/src/Result/ResultTrait.php b/src/Result/ResultTrait.php
index efb79773..76a807e5 100644
--- a/src/Result/ResultTrait.php
+++ b/src/Result/ResultTrait.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Result;
 
 use PhpSchool\PhpWorkshop\Check\CheckInterface;
diff --git a/src/Result/Success.php b/src/Result/Success.php
index 2e573d17..b4486a9d 100644
--- a/src/Result/Success.php
+++ b/src/Result/Success.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Result;
 
 use PhpSchool\PhpWorkshop\Check\CheckInterface;
diff --git a/src/Result/SuccessInterface.php b/src/Result/SuccessInterface.php
index 3b72ea96..88b3cc10 100644
--- a/src/Result/SuccessInterface.php
+++ b/src/Result/SuccessInterface.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Result;
 
 /**
diff --git a/src/ResultAggregator.php b/src/ResultAggregator.php
index dadeb624..c73407da 100644
--- a/src/ResultAggregator.php
+++ b/src/ResultAggregator.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop;
 
 use ArrayIterator;
diff --git a/src/ResultRenderer/Cgi/RequestFailureRenderer.php b/src/ResultRenderer/Cgi/RequestFailureRenderer.php
index 9cf2d1f8..0bcd9cb7 100644
--- a/src/ResultRenderer/Cgi/RequestFailureRenderer.php
+++ b/src/ResultRenderer/Cgi/RequestFailureRenderer.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\ResultRenderer\Cgi;
 
 use PhpSchool\PhpWorkshop\Result\Cgi\RequestFailure;
diff --git a/src/ResultRenderer/CgiResultRenderer.php b/src/ResultRenderer/CgiResultRenderer.php
index cf6b6266..d7ef76c8 100644
--- a/src/ResultRenderer/CgiResultRenderer.php
+++ b/src/ResultRenderer/CgiResultRenderer.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\ResultRenderer;
 
 use PhpSchool\PhpWorkshop\Result\Cgi\CgiResult;
diff --git a/src/ResultRenderer/Cli/RequestFailureRenderer.php b/src/ResultRenderer/Cli/RequestFailureRenderer.php
index 0bab93d1..19f807eb 100644
--- a/src/ResultRenderer/Cli/RequestFailureRenderer.php
+++ b/src/ResultRenderer/Cli/RequestFailureRenderer.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\ResultRenderer\Cli;
 
 use PhpSchool\PhpWorkshop\Result\Cli\RequestFailure;
diff --git a/src/ResultRenderer/CliResultRenderer.php b/src/ResultRenderer/CliResultRenderer.php
index 23a08110..a434c345 100644
--- a/src/ResultRenderer/CliResultRenderer.php
+++ b/src/ResultRenderer/CliResultRenderer.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\ResultRenderer;
 
 use PhpSchool\PhpWorkshop\Result\Cli\CliResult;
diff --git a/src/ResultRenderer/ComparisonFailureRenderer.php b/src/ResultRenderer/ComparisonFailureRenderer.php
index 7a3c6504..e0c39d23 100644
--- a/src/ResultRenderer/ComparisonFailureRenderer.php
+++ b/src/ResultRenderer/ComparisonFailureRenderer.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\ResultRenderer;
 
 use PhpSchool\PhpWorkshop\Result\Cli\RequestFailure;
diff --git a/src/ResultRenderer/FailureRenderer.php b/src/ResultRenderer/FailureRenderer.php
index e141a6e7..45797c1c 100644
--- a/src/ResultRenderer/FailureRenderer.php
+++ b/src/ResultRenderer/FailureRenderer.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\ResultRenderer;
 
 use PhpSchool\PhpWorkshop\Result\Failure;
diff --git a/src/ResultRenderer/FunctionRequirementsFailureRenderer.php b/src/ResultRenderer/FunctionRequirementsFailureRenderer.php
index 2d28e89e..fb75f66e 100644
--- a/src/ResultRenderer/FunctionRequirementsFailureRenderer.php
+++ b/src/ResultRenderer/FunctionRequirementsFailureRenderer.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\ResultRenderer;
 
 use PhpSchool\PhpWorkshop\Result\FunctionRequirementsFailure;
diff --git a/src/ResultRenderer/ResultRendererInterface.php b/src/ResultRenderer/ResultRendererInterface.php
index 1e4d4315..4a8c87d8 100644
--- a/src/ResultRenderer/ResultRendererInterface.php
+++ b/src/ResultRenderer/ResultRendererInterface.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\ResultRenderer;
 
 /**
diff --git a/src/ResultRenderer/ResultsRenderer.php b/src/ResultRenderer/ResultsRenderer.php
index 33686ee4..2aff91ba 100644
--- a/src/ResultRenderer/ResultsRenderer.php
+++ b/src/ResultRenderer/ResultsRenderer.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\ResultRenderer;
 
 use Colors\Color;
diff --git a/src/Solution/DirectorySolution.php b/src/Solution/DirectorySolution.php
index de8cf91f..ae4b4597 100644
--- a/src/Solution/DirectorySolution.php
+++ b/src/Solution/DirectorySolution.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Solution;
 
 use InvalidArgumentException;
diff --git a/src/Solution/SingleFileSolution.php b/src/Solution/SingleFileSolution.php
index 7611c983..151b9b8a 100644
--- a/src/Solution/SingleFileSolution.php
+++ b/src/Solution/SingleFileSolution.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Solution;
 
 use InvalidArgumentException;
diff --git a/src/Solution/SolutionFile.php b/src/Solution/SolutionFile.php
index 47f5d988..fe4af028 100644
--- a/src/Solution/SolutionFile.php
+++ b/src/Solution/SolutionFile.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Solution;
 
 use InvalidArgumentException;
diff --git a/src/Solution/SolutionInterface.php b/src/Solution/SolutionInterface.php
index 5df660c1..890eb34c 100644
--- a/src/Solution/SolutionInterface.php
+++ b/src/Solution/SolutionInterface.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Solution;
 
 /**
diff --git a/src/UserState.php b/src/UserState.php
index 33b1b0d2..4f53aea9 100644
--- a/src/UserState.php
+++ b/src/UserState.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop;
 
 use PhpSchool\PhpWorkshop\Exception\ExerciseNotAssignedException;
diff --git a/src/UserStateSerializer.php b/src/UserStateSerializer.php
index c9e5adce..ab9efce8 100644
--- a/src/UserStateSerializer.php
+++ b/src/UserStateSerializer.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop;
 
 /**
diff --git a/src/Utils/ArrayObject.php b/src/Utils/ArrayObject.php
index 62424202..52595e3f 100644
--- a/src/Utils/ArrayObject.php
+++ b/src/Utils/ArrayObject.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Utils;
 
 use ArrayIterator;
diff --git a/src/Utils/Collection.php b/src/Utils/Collection.php
index 2bb60621..dd5719a4 100644
--- a/src/Utils/Collection.php
+++ b/src/Utils/Collection.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Utils;
 
 /**
diff --git a/src/Utils/RequestRenderer.php b/src/Utils/RequestRenderer.php
index 825ee5f7..d1b7e936 100644
--- a/src/Utils/RequestRenderer.php
+++ b/src/Utils/RequestRenderer.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop\Utils;
 
 use Psr\Http\Message\RequestInterface;
diff --git a/src/WorkshopType.php b/src/WorkshopType.php
index 6cf6c93e..655e5a9c 100644
--- a/src/WorkshopType.php
+++ b/src/WorkshopType.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpSchool\PhpWorkshop;
 
 use MyCLabs\Enum\Enum;
diff --git a/src/functions.php b/src/functions.php
index d792efab..a9eb52a7 100644
--- a/src/functions.php
+++ b/src/functions.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 if (!function_exists('mb_str_pad')) {
 
     /**
diff --git a/test/Check/DatabaseCheckTest.php b/test/Check/DatabaseCheckTest.php
index 779836f3..4413912b 100644
--- a/test/Check/DatabaseCheckTest.php
+++ b/test/Check/DatabaseCheckTest.php
@@ -109,7 +109,7 @@ public function testIfPDOThrowsExceptionItCleansUp(): void
 
         $refProp = new ReflectionProperty(DatabaseCheck::class, 'userDsn');
         $refProp->setAccessible(true);
-        $refProp->setValue($this->check, null);
+        $refProp->setValue($this->check, 'notvaliddsn');
 
         try {
             $this->check->attach($eventDispatcher);
diff --git a/test/CommandRouterTest.php b/test/CommandRouterTest.php
index a1b18bc2..2629c02c 100644
--- a/test/CommandRouterTest.php
+++ b/test/CommandRouterTest.php
@@ -49,7 +49,7 @@ public function testRouteCommandWithNoArgsFromArrayUsesDefaultCommand(): void
 
         $mock->expects($this->once())
             ->method('__invoke')
-            ->willReturn(true);
+            ->willReturn(1);
 
         $c = $this->createMock(ContainerInterface::class);
         $eventDispatcher = $this->createMock(EventDispatcher::class);
@@ -68,7 +68,7 @@ public function testRouteCommandWithNoArgsFromArgVUsesDefaultCommand(): void
 
         $mock->expects($this->once())
             ->method('__invoke')
-            ->willReturn(true);
+            ->willReturn(1);
 
         $c = $this->createMock(ContainerInterface::class);
         $eventDispatcher = $this->createMock(EventDispatcher::class);
@@ -138,7 +138,7 @@ public function testRouteCommandWithArgs(): void
                     && $input->getArgument('exercise') === 'some-exercise'
                     && $input->getArgument('program') === 'program.php';
             }))
-            ->willReturn(true);
+            ->willReturn(1);
 
         $c = $this->createMock(ContainerInterface::class);
         $eventDispatcher = $this->createMock(EventDispatcher::class);
@@ -234,7 +234,7 @@ public function testCallableFromContainer(): void
                     && $input->getArgument('exercise') === 'some-exercise'
                     && $input->getArgument('program') === 'program.php';
             }))
-            ->willReturn(true);
+            ->willReturn(1);
 
         $c
             ->expects($this->once())
@@ -313,7 +313,7 @@ public function testRouteCommandSpeltIncorrectlyStillRoutes(): void
                 && $input->getArgument('exercise') === 'some-exercise'
                 && $input->getArgument('program') === 'program.php';
             }))
-            ->willReturn(true);
+            ->willReturn(1);
 
         $c = $this->createMock(ContainerInterface::class);
         $eventDispatcher = $this->createMock(EventDispatcher::class);
@@ -358,7 +358,7 @@ public function testRouteCommandWithOptionalArgument(): void
                     })
                 ]
             )
-            ->willReturnOnConsecutiveCalls(true, true);
+            ->willReturnOnConsecutiveCalls(1, null, 1);
 
         $c = $this->createMock(ContainerInterface::class);
         $eventDispatcher = $this->createMock(EventDispatcher::class);