When I use PHP 5.5, top of any page I have a red warning:
mysql_connect(): The mysql extension is deprecated and will be removed in the future: use mysqli or PDO instead [include/db.inc.php:77]

See any function for the "mysql" extension, for example http://php.net/manual/en/function.mysql-connect.php

PHP 5.5 is not released yet (as for 2012-12-22), only 5.5.0 Alpha2 is available, but would be good to be ready to 5.5.0 release.