diff --git a/composer.json b/composer.json
index 36c3e2b..e652261 100644
--- a/composer.json
+++ b/composer.json
@@ -32,7 +32,10 @@
         "source": "https://github.com/pear/Image_Color"
     },
     "type": "library",
+    "require" : {
+        "php": ">=5.6, < 8.0"
+    },
     "require-dev": {
-        "phpunit/phpunit": "*"
+        "yoast/phpunit-polyfills": "^2.0"
     }
 }
diff --git a/package.xml b/package.xml
index 039c626..acb8939 100644
--- a/package.xml
+++ b/package.xml
@@ -39,7 +39,6 @@ Add to Packagist
  <contents>
   <dir baseinstalldir="/" name="/">
    <file baseinstalldir="/" md5sum="b585dd216ae0d49b5db73193bb18d1aa" name="/Image/Color.php" role="php" />
-   <file baseinstalldir="/" md5sum="0b920a78c4d2dfe149046c9f1b39fe78" name="/tests/AllTests.php" role="php" />
    <file baseinstalldir="/" md5sum="885cfef95a516203992eb5b55aa6ef76" name="/tests/ColorTest.php" role="php" />
    <file baseinstalldir="/" md5sum="77817ffa40058270728fbaf274499748" name="README" role="data" />
   </dir>
diff --git a/tests/AllTests.php b/tests/AllTests.php
deleted file mode 100644
index 80d8f27..0000000
--- a/tests/AllTests.php
+++ /dev/null
@@ -1,27 +0,0 @@
-<?php
-if (!defined('PHPUnit_MAIN_METHOD')) {
-    define('PHPUnit_MAIN_METHOD', 'AllTests::main');
-}
-
-require_once 'PHPUnit/Autoload.php';
-
-require_once 'ColorTest.php';
-
-class AllTests {
-    public static function main() {
-        PHPUnit_TextUI_TestRunner::run(self::suite());
-    }
-
-    public static function suite() {
-        $suite = new PHPUnit_Framework_TestSuite('Color');
-        $suite->addTestSuite('ColorTest');
-
-        return $suite;
-    }
-}
-
-if (PHPUnit_MAIN_METHOD == 'AllTests::main') {
-    AllTests::main();
-}
-
-?>
diff --git a/tests/ColorTest.php b/tests/ColorTest.php
index 8f6b1df..b22b8f0 100644
--- a/tests/ColorTest.php
+++ b/tests/ColorTest.php
@@ -7,17 +7,23 @@
  * @copyright 2005
  */
 
-require_once 'PHPUnit/Framework/TestCase.php';
+// As we want to run on PHP < 7.1,
+// we can't use return type declaration in fixtures.
+// Therefore we use PHPUnitPolyFills snakecase fixtures set_up/tear_down
+// instead of setUp/tearDown.
+// See https://github.com/Yoast/PHPUnit-Polyfills?tab=readme-ov-file#option-1-yoastphpunitpolyfillstestcasestestcase
+use Yoast\PHPUnitPolyfills\TestCases\TestCase;
+
 require_once 'Image/Color.php';
 
-class ColorTest extends PHPUnit_Framework_TestCase {
+class ColorTest extends Yoast\PHPUnitPolyfills\TestCases\TestCase {
     var $color;
 
 
-    function setUp() {
+    function set_up() {
         $this->color = new Image_Color();
     }
-    function tearDown() {
+    function tear_down() {
         unset($this->color);
     }
 
@@ -37,13 +43,13 @@ function testSetWebsafe() {
     function testGetGetRange_DefaultParam() {
         $this->color->setColors('#ffffff', '#000000');
         $result = $this->color->getRange();
-        $this->assertInternalType('array', $result);
+        $this->assertIsArray($result);
         $this->assertEquals(2, count($result));
     }
     function testGetGetRange_Param5() {
         $this->color->setColors('#ffffff', '#000000');
         $result = $this->color->getRange(5);
-        $this->assertInternalType('array', $result);
+        $this->assertIsArray($result);
         $this->assertEquals(5, count($result));
     }