diff --git a/composer.json b/composer.json
index 860eaa4..d66f2a1 100644
--- a/composer.json
+++ b/composer.json
@@ -8,15 +8,16 @@
             "email": "wouter@wouterj.nl"
         }
     ],
-
     "require": {
-        "php": "^7.0"
+        "php": "^8.0"
     },
-
     "autoload": {
         "psr-4": { "WouterJ\\Peg\\": "src/" }
     },
     "autoload-dev": {
         "psr-4": { "WouterJ\\Peg\\": "tests/" }
+    },
+    "require-dev": {
+        "phpunit/phpunit": "^9.6"
     }
 }
diff --git a/src/PegGrammar.php b/src/PegGrammar.php
index fa9e821..a924b0a 100644
--- a/src/PegGrammar.php
+++ b/src/PegGrammar.php
@@ -55,7 +55,7 @@ public function __construct()
                     return $nested[0];
                 }
 
-                return ['choice', array_merge([$nested[0]], array_map('next', $nested[1]))];
+                return ['choice', array_merge([$nested[0]], array_map(fn($param) => next($param), $nested[1]))];
             }),
             // Sequence <- Prefix*
             new Definition('Sequence', ['repeat', ['identifier', 'Prefix']], function ($nested) {
@@ -190,7 +190,7 @@ public function __construct()
                     ['identifier', 'Spacing'],
                 ]],
             ]], function ($nested) {
-                return ['literal', implode('', array_map('next', $nested[1]))];
+                return ['literal', implode('', array_map(fn($param) => next($param), $nested[1]))];
             }),
             // Class <- ’[’ (!’]’ Range)* ’]’ Spacing
             new Definition('Class', ['sequence', [
diff --git a/tests/GrammarTest.php b/tests/GrammarTest.php
index fc7143b..89d639e 100644
--- a/tests/GrammarTest.php
+++ b/tests/GrammarTest.php
@@ -11,10 +11,12 @@
 
 namespace WouterJ\Peg;
 
+use PHPUnit\Framework\TestCase;
+
 /**
  * @author Wouter de Jong <wouter@wouterj.nl>
  */
-class GrammarTest extends \PHPUnit_Framework_TestCase
+class GrammarTest extends TestCase
 {
     public function testPredicates()
     {
diff --git a/tests/ParserTest.php b/tests/ParserTest.php
index e44119a..49b965a 100644
--- a/tests/ParserTest.php
+++ b/tests/ParserTest.php
@@ -12,11 +12,12 @@
 namespace WouterJ\Peg;
 
 use WouterJ\Peg\Exception\DefinitionException;
+use PHPUnit\Framework\TestCase;
 
 /**
  * @author Wouter de Jong <wouter@wouterj.nl>
  */
-class ParserTest extends \PHPUnit_Framework_TestCase
+class ParserTest extends TestCase
 {
     public function testLiteral()
     {
@@ -187,12 +188,11 @@ public function testActions()
         $this->assertSame(12, $parser->parse('Int', '12')->value());
     }
 
-    /**
-     * @expectedException \WouterJ\Peg\Exception\DefinitionException
-     * @expectedExceptionMessage did you mean one of these `Everything`?
-     */
     public function testUnknownDefinition()
     {
+        $this->expectExceptionMessage("did you mean one of these `Everything`?");
+        $this->expectException(\WouterJ\Peg\Exception\DefinitionException::class);
+
         $parser = new Parser([
             new Definition('Everything', ['any']),
             new Definition('Digit', ['characterClass', '0-9']),
@@ -201,12 +201,11 @@ public function testUnknownDefinition()
         $parser->parse('Everyting', 'a');
     }
 
-    /**
-     * @expectedException \WouterJ\Peg\Exception\DefinitionException
-     * @expectedExceptionMessageRegExp /^Invalid definition `Foo`: Undefined operator `undefined`\./
-     */
     public function testInvalidDefinition()
     {
+        $this->expectExceptionMessageMatches("/^Invalid definition `Foo`: Undefined operator `undefined`\./");
+        $this->expectException(\WouterJ\Peg\Exception\DefinitionException::class);
+
         $parser = new Parser([
             new Definition('Mine', ['identifier', 'Custom']),
             new Definition('Custom', ['identifier', 'Foo']),
diff --git a/tests/PegGrammarTest.php b/tests/PegGrammarTest.php
index f4e7dfb..313597e 100644
--- a/tests/PegGrammarTest.php
+++ b/tests/PegGrammarTest.php
@@ -11,10 +11,12 @@
 
 namespace WouterJ\Peg;
 
+use PHPUnit\Framework\TestCase;
+
 /**
  * @author Wouter de Jong <wouter@wouterj.nl>
  */
-class PegGrammarTest extends \PHPUnit_Framework_TestCase
+class PegGrammarTest extends TestCase
 {
     /** @dataProvider getGrammars */
     public function testGrammar($filePath)