diff --git a/.github/workflows/test-and-deploy.yml b/.github/workflows/test-and-deploy.yml
index bb5914c..1f903e5 100644
--- a/.github/workflows/test-and-deploy.yml
+++ b/.github/workflows/test-and-deploy.yml
@@ -17,7 +17,7 @@ jobs:
     timeout-minutes: 20
     strategy:
       matrix:
-        php: [ 5.6, 7.0, 7.1, 7.2, 7.3, 7.4 ]
+        php: [ '7.3', '7.4', '8.0', '8.1' ]
         dependencies:
           - "lowest"
           - "highest"
@@ -56,7 +56,7 @@ jobs:
       - name: Setup PHP
         uses: shivammathur/setup-php@2.15.0
         with:
-          php-version: '7.4'
+          php-version: '8.1'
         id: php
 
       - name: Build Release Artifacts
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 6d1425b..c587736 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -47,7 +47,7 @@ We welcome direct contributions to the php-http-client code base. Thank you!
 
 ##### Prerequisites #####
 
-- PHP 5.6 or 7.0
+- PHP version 7.3, 7.4, 8.0, or 8.1
 - [Composer](https://getcomposer.org/)
 
 ##### Initial setup: #####
diff --git a/Dockerfile b/Dockerfile
index da1ef10..2261ad9 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,4 +1,4 @@
-FROM php:7.1-apache
+FROM php:8.1
 
 ARG sendgrid_apikey
 ENV SENDGRID_API_KEY=$sendgrid_apikey
@@ -7,7 +7,8 @@ COPY . /var/www/client
 WORKDIR /var/www/client
 
 RUN apt-get update && \
-    apt-get install -y git zip zlib1g-dev && docker-php-ext-install zip
+    apt-get install -y git libzip-dev && \
+    docker-php-ext-install zip
 RUN curl --silent --show-error https://getcomposer.org/installer | php
 
 RUN php composer.phar install
diff --git a/Makefile b/Makefile
index f14433b..db07641 100644
--- a/Makefile
+++ b/Makefile
@@ -7,7 +7,7 @@ ci-install: clean
 	composer install --no-dev
 
 install: clean
-	composer install --no-suggest --no-scripts --no-progress --no-interaction
+	composer install --no-scripts --no-progress --no-interaction
 
 test: install
 	vendor/bin/phpunit test/unit
diff --git a/README.md b/README.md
index 9ef4b34..e86f3a4 100644
--- a/README.md
+++ b/README.md
@@ -27,7 +27,7 @@ All updates to this library are documented in our [CHANGELOG](CHANGELOG.md).
 
 ## Prerequisites
 
-- PHP version 5.6, 7.0, 7.1, 7.2, 7.3, or 7.4
+- PHP version 7.3, 7.4, 8.0, or 8.1
 
 ## Install with Composer
 
diff --git a/UPGRADE.md b/UPGRADE.md
new file mode 100644
index 0000000..eb3b944
--- /dev/null
+++ b/UPGRADE.md
@@ -0,0 +1,8 @@
+# Upgrade Guide
+
+_MAJOR version bumps will have upgrade notes posted here._
+
+[2022-05-04] 3.x.x to 4.x.x
+---------------------------
+
+### CHANGED - Drop support for PHP versions 5.6, 7.0, 7.1, and 7.2 which are EOL.
diff --git a/composer.json b/composer.json
index 0c637fa..2d0ead0 100644
--- a/composer.json
+++ b/composer.json
@@ -16,14 +16,13 @@
         }
     ],
     "require": {
-        "php": ">=5.6",
+        "php": ">=7.3",
         "ext-curl": "*",
         "ext-json": "*",
         "ext-mbstring": "*"
     },
     "require-dev": {
-        "phpunit/phpunit": "^5.7 || ^6.5",
-        "sebastian/version": "^1.0.6",
+        "phpunit/phpunit": "^9",
         "squizlabs/php_codesniffer": "~2.0",
         "friendsofphp/php-cs-fixer": "^2.16"
     },
diff --git a/test/unit/ClientTest.php b/test/unit/ClientTest.php
index 41f2ca8..cfea449 100644
--- a/test/unit/ClientTest.php
+++ b/test/unit/ClientTest.php
@@ -15,7 +15,7 @@ class ClientTest extends TestCase
     /** @var array */
     private $headers;
 
-    protected function setUp()
+    protected function setUp(): void
     {
         $this->host = 'https://localhost:4010';
         $this->headers = [
@@ -27,13 +27,11 @@ protected function setUp()
 
     public function testConstructor()
     {
-        $this->assertAttributeEquals($this->host, 'host', $this->client);
-        $this->assertAttributeEquals($this->headers, 'headers', $this->client);
-        $this->assertAttributeEquals('/v3', 'version', $this->client);
-        $this->assertAttributeEquals([], 'path', $this->client);
-        $this->assertAttributeEquals([], 'curlOptions', $this->client);
-        $this->assertAttributeEquals(false, 'retryOnLimit', $this->client);
-        $this->assertAttributeEquals(['get', 'post', 'patch', 'put', 'delete'], 'methods', $this->client);
+        $this->assertEquals($this->host, $this->client->getHost());
+        $this->assertEquals($this->headers, $this->client->getHeaders());
+        $this->assertEquals('/v3', $this->client->getVersion());
+        $this->assertEquals([], $this->client->getPath());
+        $this->assertEquals([], $this->client->getCurlOptions());
     }
 
     public function test_()
@@ -42,34 +40,34 @@ public function test_()
         $client->setCurlOptions(['foo' => 'bar']);
         $client = $client->_('test');
 
-        $this->assertAttributeEquals(['test'], 'path', $client);
-        $this->assertAttributeEquals(['foo' => 'bar'], 'curlOptions', $client);
+        $this->assertEquals(['test'], $client->getPath());
+        $this->assertEquals(['foo' => 'bar'], $client->getCurlOptions());
     }
 
     public function test__call()
     {
         $client = $this->client->get();
-        $this->assertAttributeEquals('https://localhost:4010/v3/', 'url', $client);
+        $this->assertEquals('https://localhost:4010/v3/', $client->url);
 
         $queryParams = ['limit' => 100, 'offset' => 0];
         $client = $this->client->get(null, $queryParams);
-        $this->assertAttributeEquals('https://localhost:4010/v3/?limit=100&offset=0', 'url', $client);
+        $this->assertEquals('https://localhost:4010/v3/?limit=100&offset=0', $client->url);
 
         $requestBody = ['name' => 'A New Hope'];
         $client = $this->client->get($requestBody);
-        $this->assertAttributeEquals($requestBody, 'requestBody', $client);
+        $this->assertEquals($requestBody, $client->requestBody);
 
         $requestHeaders = ['X-Mock: 200'];
         $client = $this->client->get(null, null, $requestHeaders);
-        $this->assertAttributeEquals($requestHeaders, 'requestHeaders', $client);
+        $this->assertEquals($requestHeaders, $client->requestHeaders);
 
         $client = $this->client->version('/v4');
-        $this->assertAttributeEquals('/v4', 'version', $client);
+        $this->assertEquals('/v4', $client->getVersion());
 
         $client = $this->client->path_to_endpoint();
-        $this->assertAttributeEquals(['path_to_endpoint'], 'path', $client);
+        $this->assertEquals(['path_to_endpoint'], $client->getPath());
         $client = $client->one_more_segment();
-        $this->assertAttributeEquals(['path_to_endpoint', 'one_more_segment'], 'path', $client);
+        $this->assertEquals(['path_to_endpoint', 'one_more_segment'], $client->getPath());
     }
 
     public function testGetHost()
@@ -212,7 +210,7 @@ public function testThrowExceptionOnInvalidCall()
     public function testMakeRequestWithUntrustedRootCert()
     {
         $this->expectException(InvalidRequest::class);
-        $this->expectExceptionMessageRegExp('/certificate/i');
+        $this->expectExceptionMessageMatches('/certificate/i');
 
         $client = new Client('https://untrusted-root.badssl.com/');
         $client->makeRequest('GET', 'https://untrusted-root.badssl.com/');
@@ -223,12 +221,12 @@ public function testFormRepeatUrlArgs()
         $client = new Client('https://localhost:4010');
 
         $testParams = [
-          'thing' => 'stuff',
-          'foo' => [
-            'bar',
-            'bat',
-            'baz',
-          ],
+            'thing' => 'stuff',
+            'foo' => [
+                'bar',
+                'bat',
+                'baz',
+            ],
         ];
         $result = $this->callMethod($client, 'buildUrl', [$testParams]);
         $this->assertEquals($result, 'https://localhost:4010/?thing=stuff&foo=bar&foo=bat&foo=baz');
diff --git a/test/unit/MockClient.php b/test/unit/MockClient.php
index ae7ed79..b1c897a 100644
--- a/test/unit/MockClient.php
+++ b/test/unit/MockClient.php
@@ -6,9 +6,9 @@
 
 class MockClient extends Client
 {
-    protected $requestBody;
-    protected $requestHeaders;
-    protected $url;
+    public $requestBody;
+    public $requestHeaders;
+    public $url;
 
     public function makeRequest($method, $url, $requestBody = null, $requestHeaders = null, $retryOnLimit = false)
     {
diff --git a/test/unit/ResponseTest.php b/test/unit/ResponseTest.php
index 3366ec6..9f83b89 100644
--- a/test/unit/ResponseTest.php
+++ b/test/unit/ResponseTest.php
@@ -2,8 +2,8 @@
 
 namespace SendGrid\Test;
 
-use SendGrid\Response;
 use PHPUnit\Framework\TestCase;
+use SendGrid\Response;
 
 class ResponseTest extends TestCase
 {
@@ -11,15 +11,15 @@ public function testConstructor()
     {
         $response = new Response();
 
-        $this->assertAttributeEquals(200, 'statusCode', $response);
-        $this->assertAttributeEquals('', 'body', $response);
-        $this->assertAttributeEquals([], 'headers', $response);
+        $this->assertEquals(200, $response->statusCode());
+        $this->assertEquals('', $response->body());
+        $this->assertEquals([], $response->headers());
 
         $response = new Response(201, 'test', ['Content-Encoding: gzip']);
 
-        $this->assertAttributeEquals(201, 'statusCode', $response);
-        $this->assertAttributeEquals('test', 'body', $response);
-        $this->assertAttributeEquals(['Content-Encoding: gzip'], 'headers', $response);
+        $this->assertEquals(201, $response->statusCode());
+        $this->assertEquals('test', $response->body());
+        $this->assertEquals(['Content-Encoding: gzip'], $response->headers());
     }
 
     public function testStatusCode()