diff --git a/.gitattributes b/.gitattributes
index a353f44..404fd5d 100644
--- a/.gitattributes
+++ b/.gitattributes
@@ -3,4 +3,5 @@
 /.gitignore export-ignore
 /examples/ export-ignore
 /phpunit.xml.dist export-ignore
+/phpunit.xml.legacy export-ignore
 /tests/ export-ignore
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 438fcc4..f8b1999 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -7,7 +7,7 @@ on:
 jobs:
   PHPUnit:
     name: PHPUnit (PHP ${{ matrix.php }})
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-22.04
     strategy:
       matrix:
         php:
@@ -20,10 +20,13 @@ jobs:
           - 5.4
           - 5.3
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - uses: shivammathur/setup-php@v2
         with:
           php-version: ${{ matrix.php }}
           coverage: xdebug
       - run: composer install
       - run: vendor/bin/phpunit --coverage-text
+        if: ${{ matrix.php >= 7.3 }}
+      - run: vendor/bin/phpunit --coverage-text -c phpunit.xml.legacy
+        if: ${{ matrix.php < 7.3 }}
diff --git a/README.md b/README.md
index 58eb26c..fd0774a 100644
--- a/README.md
+++ b/README.md
@@ -1,6 +1,6 @@
 # clue/sse-react
 
-[![CI status](https://github.com/clue/php-sse-react/workflows/CI/badge.svg)](https://github.com/clue/php-sse-react/actions)
+[![CI status](https://github.com/clue/php-sse-react/actions/workflows/ci.yml/badge.svg)](https://github.com/clue/php-sse-react/actions)
 [![installs on Packagist](https://img.shields.io/packagist/dt/clue/sse-react?color=blue&label=installs%20on%20Packagist)](https://packagist.org/packages/clue/sse-react)
 
 Streaming, async HTML5 Server-Sent Events server (aka. SSE or EventSource), built on top of [React PHP](http://reactphp.org/).
@@ -13,7 +13,11 @@ See the [examples](examples).
 
 ## Install
 
-The recommended way to install this library is [through composer](http://getcomposer.org). [New to composer?](http://getcomposer.org/doc/00-intro.md)
+The recommended way to install this library is [through Composer](https://getcomposer.org/).
+[New to Composer?](https://getcomposer.org/doc/00-intro.md)
+
+Once released, this project will follow [SemVer](https://semver.org/).
+At the moment, this will install the latest development version:
 
 ```JSON
 {
@@ -23,21 +27,25 @@ The recommended way to install this library is [through composer](http://getcomp
 }
 ```
 
+This project aims to run on any platform and thus does not require any PHP
+extensions and supports running on legacy PHP 5.3 through PHP 7.3.
+It's *highly recommended to use the latest supported PHP version* for this project.
+
 ## Tests
 
 To run the test suite, you first need to clone this repo and then install all
-dependencies [through Composer](http://getcomposer.org):
+dependencies [through Composer](http://getcomposer.org/):
 
 ```bash
-$ composer install
+composer install
 ```
 
 To run the test suite, go to the project root and run:
 
 ```bash
-$ php vendor/bin/phpunit
+php vendor/bin/phpunit
 ```
 
 ## License
 
-MIT
+MIT, see [LICENSE file](LICENSE).
diff --git a/composer.json b/composer.json
index 5da0c8d..c6c7278 100644
--- a/composer.json
+++ b/composer.json
@@ -10,9 +10,6 @@
             "email": "christian@lueck.tv"
         }
     ],
-    "autoload": {
-        "psr-4": { "Clue\\React\\Sse\\": "src/" }
-    },
     "require": {
         "php": ">=5.3",
         "react/event-loop": "^1.0",
@@ -21,6 +18,16 @@
     },
     "require-dev": {
         "clue/redis-react": "^2.4",
-        "phpunit/phpunit": "^5.0 || ^4.8"
+        "phpunit/phpunit": "^9.6 || ^7.5 || ^4.8.36"
+    },
+    "autoload": {
+        "psr-4": {
+            "Clue\\React\\Sse\\": "src/"
+        }
+    },
+    "autoload-dev": {
+        "psr-4": {
+            "Clue\\React\\Tests\\Sse\\": "tests/"
+        }
     }
 }
diff --git a/phpunit.xml.dist b/phpunit.xml.dist
index 4633355..2c1a022 100644
--- a/phpunit.xml.dist
+++ b/phpunit.xml.dist
@@ -1,19 +1,23 @@
 <?xml version="1.0" encoding="UTF-8"?>
 
-<phpunit bootstrap="tests/bootstrap.php"
+<!-- PHPUnit configuration file with new format for PHPUnit 9.6+ -->
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/9.6/phpunit.xsd"
+         bootstrap="vendor/autoload.php"
+         cacheResult="false"
          colors="true"
-         convertErrorsToExceptions="true"
-         convertNoticesToExceptions="true"
-         convertWarningsToExceptions="true"
->
+         convertDeprecationsToExceptions="true">
     <testsuites>
         <testsuite name="SSE Test Suite">
             <directory>./tests/</directory>
         </testsuite>
     </testsuites>
-    <filter>
-        <whitelist>
+    <coverage>
+        <include>
             <directory>./src/</directory>
-        </whitelist>
-    </filter>
-</phpunit>
\ No newline at end of file
+        </include>
+    </coverage>
+    <php>
+        <ini name="error_reporting" value="-1" />
+    </php>
+</phpunit>
diff --git a/phpunit.xml.legacy b/phpunit.xml.legacy
new file mode 100644
index 0000000..c9978bd
--- /dev/null
+++ b/phpunit.xml.legacy
@@ -0,0 +1,21 @@
+<?xml version="1.0" encoding="UTF-8"?>
+
+<!-- PHPUnit configuration file with old format for legacy PHPUnit -->
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/4.8/phpunit.xsd"
+         bootstrap="vendor/autoload.php"
+         colors="true">
+    <testsuites>
+        <testsuite name="SSE Test Suite">
+            <directory>./tests/</directory>
+        </testsuite>
+    </testsuites>
+    <filter>
+        <whitelist>
+            <directory>./src/</directory>
+        </whitelist>
+    </filter>
+    <php>
+        <ini name="error_reporting" value="-1" />
+    </php>
+</phpunit>
diff --git a/tests/BufferedChannelTest.php b/tests/BufferedChannelTest.php
index 2cd4cdb..639bd3f 100644
--- a/tests/BufferedChannelTest.php
+++ b/tests/BufferedChannelTest.php
@@ -1,5 +1,7 @@
 <?php
 
+namespace Clue\React\Tests\Sse;
+
 use Clue\React\Sse\BufferedChannel;
 
 class BufferedChannelTest extends TestCase
diff --git a/tests/EncoderTest.php b/tests/EncoderTest.php
index 17751c3..ac47ec9 100644
--- a/tests/EncoderTest.php
+++ b/tests/EncoderTest.php
@@ -1,12 +1,17 @@
 <?php
 
+namespace Clue\React\Tests\Sse;
+
 use Clue\React\Sse\Encoder;
 
 class EncoderTest extends TestCase
 {
     private $encoder;
 
-    public function setUp()
+    /**
+     * @before
+     */
+    public function setUpEncoder()
     {
         $this->encoder = new Encoder();
     }
diff --git a/tests/TestCase.php b/tests/TestCase.php
new file mode 100644
index 0000000..66af970
--- /dev/null
+++ b/tests/TestCase.php
@@ -0,0 +1,32 @@
+<?php
+
+namespace Clue\React\Tests\Sse;
+
+use PHPUnit\Framework\TestCase as BaseTestCase;
+
+require __DIR__ . '/../vendor/autoload.php';
+
+class TestCase extends BaseTestCase
+{
+    protected function expectCallableOnce()
+    {
+        $mock = $this->createCallableMock();
+
+        $mock
+            ->expects($this->once())
+            ->method('__invoke');
+
+        return $mock;
+    }
+
+    protected function createCallableMock()
+    {
+        if (method_exists('PHPUnit\Framework\MockObject\MockBuilder', 'addMethods')) {
+            // PHPUnit 9+
+            return $this->getMockBuilder('stdClass')->addMethods(array('__invoke'))->getMock();
+        } else {
+            // legacy PHPUnit 4 - PHPUnit 8
+            return $this->getMockBuilder('stdClass')->setMethods(array('__invoke'))->getMock();
+        }
+    }
+}
diff --git a/tests/bootstrap.php b/tests/bootstrap.php
deleted file mode 100644
index 572fa88..0000000
--- a/tests/bootstrap.php
+++ /dev/null
@@ -1,34 +0,0 @@
-<?php
-
-require __DIR__ . '/../vendor/autoload.php';
-
-PHPUnit_Framework_Error_Deprecated::$enabled = false;
-
-class TestCase extends PHPUnit_Framework_TestCase
-{
-    protected function expectCallableOnce()
-    {
-        $mock = $this->createCallableMock();
-
-        $mock
-            ->expects($this->once())
-            ->method('__invoke');
-
-        return $mock;
-    }
-
-    /**
-     * @link https://github.com/reactphp/react/blob/master/tests/React/Tests/Socket/TestCase.php (taken from reactphp/react)
-     */
-    protected function createCallableMock()
-    {
-        return $this->getMockBuilder('CallableStub')->getMock();
-    }
-}
-
-class CallableStub
-{
-    public function __invoke()
-    {
-    }
-}