[link|http://www.php.net/manual/en/migration5.incompatible.php|PHP.net incompatible changes between 4 and 5] plus the big changes to the way the object system works that Drew mentioned.

In the end, the changes probably won't have much impact on a simple web site. But if your building a big web application, the better OO in PHP 5 gives it a big advantage.

Hope you find something that pays.

Jay