diff --git a/.github/workflows/php.yml b/.github/workflows/php.yml
index d07acac..5e896c1 100644
--- a/.github/workflows/php.yml
+++ b/.github/workflows/php.yml
@@ -1,6 +1,9 @@
 name: CI
 
-on: [push]
+on:
+  push:
+  pull_request:
+  workflow_dispatch:
 
 jobs:
 
@@ -9,7 +12,7 @@ jobs:
 
     strategy:
       matrix:
-        php-versions: ['7.4', '8.2']
+        php-versions: ['8.1', '8.4']
 
     steps:
       - uses: actions/checkout@v2
diff --git a/composer.json b/composer.json
index 8a26477..a783e68 100644
--- a/composer.json
+++ b/composer.json
@@ -20,13 +20,13 @@
         }
     ],
     "require": {
-        "php": ">=7.1"
+        "php": ">=8.1"
     },
     "require-dev": {
-        "phpunit/phpunit": "8.5.*",
+        "phpunit/phpunit": "9.6.23",
         "psr/log": "1.0.*@dev",
-        "twig/twig": "3.7.*",
-        "phpstan/phpstan": "^1.10"
+        "phpstan/phpstan": "^1.10",
+        "twig/twig": "3.11.3"
     },
     "bin": [
         "bin/csscrush"
diff --git a/lib/CssCrush/Crush.php b/lib/CssCrush/Crush.php
index 230d607..0c1348e 100644
--- a/lib/CssCrush/Crush.php
+++ b/lib/CssCrush/Crush.php
@@ -97,7 +97,7 @@ public static function loadAssets()
         }
     }
 
-    public static function plugin($name = null, callable|null $callback = null)
+    public static function plugin($name = null, ?callable $callback = null)
     {
         static $plugins = [];
 
diff --git a/lib/CssCrush/Fragment.php b/lib/CssCrush/Fragment.php
index 563ee04..334a5ba 100644
--- a/lib/CssCrush/Fragment.php
+++ b/lib/CssCrush/Fragment.php
@@ -16,7 +16,7 @@ public function __construct($str, $options = [])
         $this->name = $options['name'];
     }
 
-    public function __invoke(array $args = null, $str = null)
+    public function __invoke(?array $args = null, $str = null)
     {
         $str = parent::__invoke($args);
 
diff --git a/lib/CssCrush/Process.php b/lib/CssCrush/Process.php
index 85d6968..06d8791 100644
--- a/lib/CssCrush/Process.php
+++ b/lib/CssCrush/Process.php
@@ -667,8 +667,8 @@ public function captureRules()
 
             preg_match($rulePatt, $this->string->raw, $ruleMatch, PREG_UNMATCHED_AS_NULL, $traceOffset);
 
-            $selector = trim($ruleMatch['selector'] ?? null);
-            $block = trim($ruleMatch['block_content'] ?? null);
+            $selector = trim($ruleMatch['selector'] ?? '');
+            $block = trim($ruleMatch['block_content'] ?? '');
             $replace = '';
 
             // If rules are nested inside we set their parent property.
@@ -688,12 +688,12 @@ public function captureRules()
                 }
             }
             else  {
-                $rule = new Rule($selector, $block, $ruleMatch['trace_token'] ?? null);
+                $rule = new Rule($selector, $block, $ruleMatch['trace_token'] ?? '');
             }
 
             $replace = $tokens->add($rule, 'r', $rule->label) . $replace;
 
-            $this->string->splice($replace, $traceOffset, strlen($ruleMatch[0]) ?? null);
+            $this->string->splice($replace, $traceOffset, strlen($ruleMatch[0]) ?? '');
         }
 
         // Flip, since we just captured rules in reverse order.
diff --git a/lib/CssCrush/StringObject.php b/lib/CssCrush/StringObject.php
index 999afc3..66a6a58 100644
--- a/lib/CssCrush/StringObject.php
+++ b/lib/CssCrush/StringObject.php
@@ -8,7 +8,7 @@
 
 class StringObject
 {
-    public string|null $raw = null;
+    public ?string $raw = null;
 
     public function __construct($str)
     {
diff --git a/lib/CssCrush/Template.php b/lib/CssCrush/Template.php
index 653f558..a459c8b 100644
--- a/lib/CssCrush/Template.php
+++ b/lib/CssCrush/Template.php
@@ -60,7 +60,7 @@ public function __construct($str)
         $this->string = $templateFunctions->apply($str);
     }
 
-    public function __invoke(array $args = null, $str = null)
+    public function __invoke(?array $args = null, $str = null)
     {
         $str = isset($str) ? $str : $this->string;
 
diff --git a/tests/unit/CssCrush/OptionsTest.php b/tests/unit/CssCrush/OptionsTest.php
index c09655b..b140f78 100644
--- a/tests/unit/CssCrush/OptionsTest.php
+++ b/tests/unit/CssCrush/OptionsTest.php
@@ -83,7 +83,7 @@ public function testSourceMaps()
         csscrush_file($this->testFile, ['source_map' => true]);
         $source_map_contents = file_get_contents("$this->testFile.crush.css.map");
 
-        $this->assertRegExp('~"version": ?3,~', $source_map_contents);
+        $this->assertMatchesRegularExpression('~"version": ?3,~', $source_map_contents);
     }
 
     public function testAdvancedMinify()
diff --git a/tests/unit/CssCrush/TokensTest.php b/tests/unit/CssCrush/TokensTest.php
index 2abc3e6..72a1ebc 100644
--- a/tests/unit/CssCrush/TokensTest.php
+++ b/tests/unit/CssCrush/TokensTest.php
@@ -32,7 +32,7 @@ public function test__construct()
     public function testCreateLabel()
     {
         $type = 's';
-        $this->assertRegExp("~^\?{$type}[a-z0-9]+\?$~", $this->tokens->createLabel($type));
+        $this->assertMatchesRegularExpression("~^\?{$type}[a-z0-9]+\?$~", $this->tokens->createLabel($type));
     }
 
     public function testAdd()
diff --git a/tests/unit/CssCrush/VersionTest.php b/tests/unit/CssCrush/VersionTest.php
index 3f8fd31..aec0fae 100644
--- a/tests/unit/CssCrush/VersionTest.php
+++ b/tests/unit/CssCrush/VersionTest.php
@@ -36,7 +36,7 @@ public function testProperties()
     public function testGitDescribe()
     {
         if ($version = Version::gitDescribe()) {
-            $this->assertRegExp('~^
+            $this->assertMatchesRegularExpression('~^
                 v
                 \d+\.
                 \d+\.