On his blog, Jaisen Methai posted about a very handy feature of PHP5 - the __autoload function.
It's one of the gems in PHP that I find to be relatively under used. It's common for PHP applications to break out classes into their own files. This becomes cumbersome when working on large projects as you wind up with numerous include/require calls for any given page.
He shows an example of its usage (loading class files dynamically from a specified directory) and mentions how it can help to make your code a lot less ugly. Comments on the post range in topics - from comparing it to include paths, how the idea was borrowed from PEAR, and how proper naming conventions can make its use even easier.