Forbid serializing a Crawler

Unserializing a Crawler instance creates DOM elements in an invalid
state, making the Crawler unusable.

src/Symfony/Component/DomCrawler/Crawler.php

@@ -311,6 +311,17 @@ class Crawler extends \SplObjectStorage
         }
     }
 
+    // Serializing and unserializing a crawler creates DOM objects in a corrupted state. DOM elements are not properly serializable.
+    public function unserialize($serialized)
+    {
+        throw new \BadMethodCallException('A Crawler cannot be serialized.');
+    }
+
+    public function serialize()
+    {
+        throw new \BadMethodCallException('A Crawler cannot be serialized.');
+    }
+
     /**
      * Returns a node given its position in the node list.
      *