If you've ever really found a time when you needed to overwrite one of PHP's bult-in functions with your own custom one (maybe a custom handler or the like), you might want to check out this post on OpenIdeasCompany.com.

The author needed to redeclare session_start in a few files to change the place the default sessions are located at (spanned across three machines). He didn't want to rewrite the application to make this change, so he used the runkit extension out of pecl (and the php-devel package) to create a solution redefining functions to match his needs.

Of course, there are some easier ways to accomplish what he needed, but it's still an interesting method if there's really just no other way.