diff --git a/src/CallbackHandler.php b/src/CallbackHandler.php
index ca12f9244..cdc50511f 100644
--- a/src/CallbackHandler.php
+++ b/src/CallbackHandler.php
@@ -43,6 +43,12 @@ class CallbackHandler
      */
     protected static $isPhp54;
 
+    /**
+     * Is pecl/weakref extension installed?
+     * @var boolean
+     */
+    protected static $hasWeakRefExtension;
+
     /**
      * Constructor
      *
@@ -76,8 +82,12 @@ protected function registerCallback($callback)
             throw new Exception\InvalidCallbackException('Invalid callback provided; not callable');
         }
 
+        if (null === self::$hasWeakRefExtension) {
+            self::$hasWeakRefExtension = class_exists('WeakRef');
+        }
+
         // If pecl/weakref is not installed, simply store the callback and return
-        if (!class_exists('WeakRef')) {
+        if (!self::$hasWeakRefExtension) {
             $this->callback = $callback;
             return;
         }