diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index 7f1d694..722fe4e 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -12,8 +12,7 @@ jobs:
     strategy:
       fail-fast: true
       matrix:
-        php: [7.0, 7.1, 7.2, 7.3, 7.4, 8.0]
-        stability: [prefer-stable]
+        php: [7.4, 8.0, 8.1, 8.2, 8.3, 8.4]
 
     name: PHP ${{ matrix.php }}
 
@@ -21,24 +20,16 @@ jobs:
       - name: Checkout code
         uses: actions/checkout@v2
 
-      - name: Cache dependencies
-        uses: actions/cache@v1
-        with:
-          path: ~/.composer/cache/files
-          key: dependencies-php-${{ matrix.php }}-composer-${{ hashFiles('composer.json') }}
-
       - name: Setup PHP
         uses: shivammathur/setup-php@v2
         with:
           php-version: ${{ matrix.php }}
           extensions: bcmath, json, mbstring, intl
+          tools: composer:v2
           coverage: none
 
-      - name: Update composer
-        run: composer self-update
-
       - name: Install dependencies
-        run: composer update --${{ matrix.stability }} --prefer-dist --no-interaction --no-progress
+        run: composer update --no-interaction --no-progress
 
       - name: Execute tests
         run: vendor/bin/phpunit --verbose
\ No newline at end of file
diff --git a/composer.json b/composer.json
index 9962efe..3b088f1 100644
--- a/composer.json
+++ b/composer.json
@@ -20,10 +20,10 @@
         }
     ],
     "require": {
-        "php": ">=7.0"
+        "php": ">=7.4"
     },
     "require-dev": {
-        "opis/string": "^1.4",
+        "opis/string": "^2.0",
         "ext-bcmath": "*",
         "ext-json": "*",
         "ext-mbstring": "*",
diff --git a/src/Formats/AbstractFormat.php b/src/Formats/AbstractFormat.php
index 4950f5b..f455ae0 100644
--- a/src/Formats/AbstractFormat.php
+++ b/src/Formats/AbstractFormat.php
@@ -38,7 +38,7 @@ protected function validateRegex($data, string $regex): bool
      * @param null $options
      * @return bool
      */
-    protected function validateFilter($data, int $filter, $options = null): bool
+    protected function validateFilter($data, int $filter, $options = 0): bool
     {
         return filter_var($data, $filter, $options) !== false;
     }
diff --git a/src/ISchema.php b/src/ISchema.php
index 0bd8ea0..4c0948a 100644
--- a/src/ISchema.php
+++ b/src/ISchema.php
@@ -36,6 +36,6 @@ public function draft(): string;
      * @param string|null $id
      * @return mixed
      */
-    public function resolve(string $id = null);
+    public function resolve(?string $id = null);
 
 }
\ No newline at end of file
diff --git a/src/IValidator.php b/src/IValidator.php
index e8b7c26..91d43fa 100644
--- a/src/IValidator.php
+++ b/src/IValidator.php
@@ -28,7 +28,7 @@ interface IValidator
      * @param ISchemaLoader|null $loader
      * @return ValidationResult
      */
-    public function schemaValidation($data, ISchema $schema, int $max_errors = 1, ISchemaLoader $loader = null): ValidationResult;
+    public function schemaValidation($data, ISchema $schema, int $max_errors = 1, ?ISchemaLoader $loader = null): ValidationResult;
 
     /**
      * @param $data
@@ -37,7 +37,7 @@ public function schemaValidation($data, ISchema $schema, int $max_errors = 1, IS
      * @param ISchemaLoader|null $loader
      * @return ValidationResult
      */
-    public function uriValidation($data, string $schema_uri, int $max_errors = 1, ISchemaLoader $loader = null): ValidationResult;
+    public function uriValidation($data, string $schema_uri, int $max_errors = 1, ?ISchemaLoader $loader = null): ValidationResult;
 
     /**
      * @param $data
@@ -46,13 +46,13 @@ public function uriValidation($data, string $schema_uri, int $max_errors = 1, IS
      * @param ISchemaLoader|null $loader
      * @return ValidationResult
      */
-    public function dataValidation($data, $schema, int $max_errors = 1, ISchemaLoader $loader = null): ValidationResult;
+    public function dataValidation($data, $schema, int $max_errors = 1, ?ISchemaLoader $loader = null): ValidationResult;
 
     /**
      * @param IFilterContainer|null $filters
      * @return IValidator
      */
-    public function setFilters(IFilterContainer $filters = null): self;
+    public function setFilters(?IFilterContainer $filters = null): self;
 
     /**
      * @return IFilterContainer|null
@@ -63,7 +63,7 @@ public function getFilters();
      * @param IFormatContainer|null $formats
      * @return IValidator
      */
-    public function setFormats(IFormatContainer $formats = null): self;
+    public function setFormats(?IFormatContainer $formats = null): self;
 
     /**
      * @return IFilterContainer|null
@@ -85,7 +85,7 @@ public function getHelper(): IValidatorHelper;
      * @param ISchemaLoader|null $loader
      * @return IValidator
      */
-    public function setLoader(ISchemaLoader $loader = null): self;
+    public function setLoader(?ISchemaLoader $loader = null): self;
 
     /**
      * @return ISchemaLoader|null
@@ -96,7 +96,7 @@ public function getLoader();
      * @param IMediaTypeContainer|null $media
      * @return IValidator
      */
-    public function setMediaType(IMediaTypeContainer $media = null): self;
+    public function setMediaType(?IMediaTypeContainer $media = null): self;
 
     /**
      * @return IMediaTypeContainer|null
diff --git a/src/IValidatorHelper.php b/src/IValidatorHelper.php
index f951eda..57ab9e4 100644
--- a/src/IValidatorHelper.php
+++ b/src/IValidatorHelper.php
@@ -61,6 +61,6 @@ public function isMultipleOf($number, $divisor): bool;
      * @param array $defaults
      * @return bool
      */
-    public function equals($a, $b, array $defaults = null): bool;
+    public function equals($a, $b, ?array $defaults = null): bool;
 
 }
\ No newline at end of file
diff --git a/src/Loaders/Memory.php b/src/Loaders/Memory.php
index 8564147..5396cdf 100644
--- a/src/Loaders/Memory.php
+++ b/src/Loaders/Memory.php
@@ -59,7 +59,7 @@ public function unregister(ISchema $schema): self
      * @param string|null $id
      * @return Memory
      */
-    public function add($data, string $id = null): self
+    public function add($data, ?string $id = null): self
     {
         if (is_string($data)) {
             $data = json_decode($data, false);
diff --git a/src/Schema.php b/src/Schema.php
index 027fc3d..f70cd08 100644
--- a/src/Schema.php
+++ b/src/Schema.php
@@ -53,7 +53,7 @@ class Schema implements ISchema
      * @param \stdClass|boolean $data
      * @param string|null $id
      */
-    public function __construct($data, string $id = null)
+    public function __construct($data, ?string $id = null)
     {
         if (is_object($data)) {
             if (property_exists($data, static::ID_PROP)) {
@@ -117,7 +117,7 @@ public function draft(): string
     /**
      * @inheritDoc
      */
-    public function resolve(string $id = null)
+    public function resolve(?string $id = null)
     {
         if ($id === null) {
             $id = $this->id;
@@ -196,7 +196,7 @@ public static function walk(array &$container, &$schema, string $id, array $path
      * @param string|null $id
      * @return Schema
      */
-    public static function fromJsonString(string $json, string $id = null): self
+    public static function fromJsonString(string $json, ?string $id = null): self
     {
         return new self(json_decode($json, false), $id);
     }
diff --git a/src/Validator.php b/src/Validator.php
index 6fa2675..1c35f66 100644
--- a/src/Validator.php
+++ b/src/Validator.php
@@ -65,11 +65,11 @@ class Validator implements IValidator
      * @param IFilterContainer|null $filters
      * @param IMediaTypeContainer|null $media
      */
-    public function __construct(IValidatorHelper $helper = null,
-                                ISchemaLoader $loader = null,
-                                IFormatContainer $formats = null,
-                                IFilterContainer $filters = null,
-                                IMediaTypeContainer $media = null)
+    public function __construct(?IValidatorHelper $helper = null,
+                                ?ISchemaLoader $loader = null,
+                                ?IFormatContainer $formats = null,
+                                ?IFilterContainer $filters = null,
+                                ?IMediaTypeContainer $media = null)
     {
         $this->helper = $helper ?? new ValidatorHelper();
         $this->formats = $formats ?? new FormatContainer();
@@ -81,7 +81,7 @@ public function __construct(IValidatorHelper $helper = null,
     /**
      * @inheritDoc
      */
-    public function schemaValidation($data, ISchema $schema, int $max_errors = 1, ISchemaLoader $loader = null): ValidationResult
+    public function schemaValidation($data, ISchema $schema, int $max_errors = 1, ?ISchemaLoader $loader = null): ValidationResult
     {
         $bag = new ValidationResult($max_errors);
 
@@ -103,7 +103,7 @@ public function schemaValidation($data, ISchema $schema, int $max_errors = 1, IS
     /**
      * @inheritDoc
      */
-    public function uriValidation($data, string $schema_uri, int $max_errors = 1, ISchemaLoader $loader = null): ValidationResult
+    public function uriValidation($data, string $schema_uri, int $max_errors = 1, ?ISchemaLoader $loader = null): ValidationResult
     {
         $schema = new stdClass();
         $schema->{'$ref'} = URI::normalize($schema_uri);
@@ -113,7 +113,7 @@ public function uriValidation($data, string $schema_uri, int $max_errors = 1, IS
     /**
      * @inheritDoc
      */
-    public function dataValidation($data, $schema, int $max_errors = 1, ISchemaLoader $loader = null): ValidationResult
+    public function dataValidation($data, $schema, int $max_errors = 1, ?ISchemaLoader $loader = null): ValidationResult
     {
         $schema = is_string($schema) ? Schema::fromJsonString($schema) : new Schema($schema);
         return $this->schemaValidation($data, $schema, $max_errors, $loader);
@@ -122,7 +122,7 @@ public function dataValidation($data, $schema, int $max_errors = 1, ISchemaLoade
     /**
      * @inheritDoc
      */
-    public function setFilters(IFilterContainer $filters = null): IValidator
+    public function setFilters(?IFilterContainer $filters = null): IValidator
     {
         $this->filters = $filters;
         return $this;
@@ -139,7 +139,7 @@ public function getFilters()
     /**
      * @inheritDoc
      */
-    public function setFormats(IFormatContainer $formats = null): IValidator
+    public function setFormats(?IFormatContainer $formats = null): IValidator
     {
         $this->formats = $formats;
         return $this;
@@ -173,7 +173,7 @@ public function getHelper(): IValidatorHelper
     /**
      * @inheritDoc
      */
-    public function setLoader(ISchemaLoader $loader = null): IValidator
+    public function setLoader(?ISchemaLoader $loader = null): IValidator
     {
         $this->loader = $loader;
         return $this;
@@ -190,7 +190,7 @@ public function getLoader()
     /**
      * @inheritDoc
      */
-    public function setMediaType(IMediaTypeContainer $media = null): IValidator
+    public function setMediaType(?IMediaTypeContainer $media = null): IValidator
     {
         $this->mediaTypes = $media;
         return $this;
@@ -544,7 +544,7 @@ protected function validateKeywords(&$document_data, &$data, array $data_pointer
      * @return bool
      */
     protected function validateCommons(/** @noinspection PhpUnusedParameterInspection */
-        &$document_data, &$data, array $data_pointer, array $parent_data_pointer, ISchema $document, $schema, ValidationResult $bag, array &$defaults = null): bool
+        &$document_data, &$data, array $data_pointer, array $parent_data_pointer, ISchema $document, $schema, ValidationResult $bag, ?array &$defaults = null): bool
     {
         $ok = true;
 
@@ -929,7 +929,7 @@ protected function validateConditionals(&$document_data, &$data, array $data_poi
      * @param array|null $defaults
      * @return bool
      */
-    protected function validateProperties(&$document_data, &$data, array $data_pointer, array $parent_data_pointer, ISchema $document, $schema, ValidationResult $bag, array $defaults = null): bool
+    protected function validateProperties(&$document_data, &$data, array $data_pointer, array $parent_data_pointer, ISchema $document, $schema, ValidationResult $bag, ?array $defaults = null): bool
     {
         $type = $this->helper->type($data, true);
         if ($type === 'null' || $type === 'boolean') {
@@ -1694,7 +1694,7 @@ protected function validateArray(&$document_data, &$data, array $data_pointer, a
      * @param array|null $defaults
      * @return bool
      */
-    protected function validateObject(&$document_data, &$data, array $data_pointer, array $parent_data_pointer, ISchema $document, $schema, ValidationResult $bag, array &$defaults = null): bool
+    protected function validateObject(&$document_data, &$data, array $data_pointer, array $parent_data_pointer, ISchema $document, $schema, ValidationResult $bag, ?array &$defaults = null): bool
     {
         $ok = true;
 
@@ -2014,7 +2014,7 @@ protected function validateObject(&$document_data, &$data, array $data_pointer,
      * @param $data
      * @param $defaults
      */
-    protected function setObjectDefaults($data, array &$defaults = null)
+    protected function setObjectDefaults($data, ?array &$defaults = null)
     {
         if (is_object($data) && $defaults) {
             foreach ($defaults as $property => $value) {
diff --git a/src/ValidatorHelper.php b/src/ValidatorHelper.php
index 6c85d4b..459d799 100644
--- a/src/ValidatorHelper.php
+++ b/src/ValidatorHelper.php
@@ -139,7 +139,7 @@ public function isMultipleOf($number, $divisor): bool
     /**
      * @inheritDoc
      */
-    public function equals($a, $b, array $defaults = null): bool
+    public function equals($a, $b, ?array $defaults = null): bool
     {
         $a_type = $this->type($a, false);
         $b_type = $this->type($b, false);
diff --git a/tests/JsonValidatorTrait.php b/tests/JsonValidatorTrait.php
index c521748..890dc84 100644
--- a/tests/JsonValidatorTrait.php
+++ b/tests/JsonValidatorTrait.php
@@ -36,7 +36,7 @@ protected function getValidator(): IValidator
         return $this->validator;
     }
 
-    protected function createValidator(ISchemaLoader $loader = null, bool $use_default = true): IValidator
+    protected function createValidator(?ISchemaLoader $loader = null, bool $use_default = true): IValidator
     {
         if ($loader === null) {
             $loader = new File( 'schema:', [__DIR__ . '/schemas']);