When PHP 5.3 was released, some older features were retired (some removed, some "deprecated", etc). And if your custom code or your addons/plugins are using those features, things will break and you'll encounter the dreaded "white screen of death". What do you need to change? Here's the list I generally follow, along with how I usually remedy it: ereg('searchtext', 'a_string', $optionalVar) becomes preg_match('/searchtext/ ...