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/