Added PHPStan rule that checks we don't use $this in closures.

phpstan/Rules/tests/AnonymousFunctionWithThisRuleTest.php

@@ -0,0 +1,28 @@
+<?php declare(strict_types = 1);
+
+namespace Composer\PHPStanRulesTests;
+
+use Composer\PHPStanRules\AnonymousFunctionWithThisRule;
+use PHPStan\Testing\RuleTestCase;
+
+/**
+ * @phpstan-extends RuleTestCase<AnonymousFunctionWithThisRule>
+ */
+final class AnonymousFunctionWithThisRuleTest extends RuleTestCase
+{
+    /**
+     * @inheritDoc
+     */
+    protected function getRule(): \PHPStan\Rules\Rule
+    {
+        return new AnonymousFunctionWithThisRule();
+    }
+
+    public function testWithThis(): void
+    {
+        $this->analyse([__DIR__ . '/data/method-with-this.php'], [
+            ['Using $this inside anonymous function is prohibited because of PHP 5.3 support.', 13],
+            ['Using $this inside anonymous function is prohibited because of PHP 5.3 support.', 17],
+        ]);
+    }
+}