Mike Knoop

// Code to be placed into your facebookapi_php5_restlib.php file.
  /**
   * Returns an array of mutual friends between the source and target
   *
   * @param int $uid1  Source UID
   * @param int $uid2  Target UID
   *
   * @return array  An array of friends
   */
  public function &friends_getMutualFriends($uid1=null, $uid2 = null) {
    return $this->call_method('facebook.friends.getMutualFriends',
        array('target_uid' => $uid1,
              'source_uid' => $uid2));

  }
// end


// Code to utilize the above call
$friends = $GLOBALS['Facebook']->api_client->friends_getMutualFriends('4', $_POST['fb_sig_user']);
$count = count($friends);

echo "You have $count friends in common.<br />";
print_r($friends);
// end