--- /dev/null
+<?php
+
+/*
+ * This file is part of Psy Shell.
+ *
+ * (c) 2012-2017 Justin Hileman
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Psy\CodeCleaner;
+
+use PhpParser\Node as Node;
+use PhpParser\Node\Expr\Assign;
+use PhpParser\Node\Expr\Variable;
+use Psy\Exception\FatalErrorException;
+
+/**
+ * Validate that the user input does not assign the `$this` variable.
+ *
+ * @author Martin HasoĊ <martin.hason@gmail.com>
+ */
+class AssignThisVariablePass extends CodeCleanerPass
+{
+ /**
+ * Validate that the user input does not assign the `$this` variable.
+ *
+ * @throws RuntimeException if the user assign the `$this` variable
+ *
+ * @param Node $node
+ */
+ public function enterNode(Node $node)
+ {
+ if ($node instanceof Assign && $node->var instanceof Variable && $node->var->name === 'this') {
+ throw new FatalErrorException('Cannot re-assign $this');
+ }
+ }
+}