In a note pointed to from PHPEverywhere this morning, a page has been made (as part of a manual appendix) to discuss the move from documentation for PHP4 to PHP5.

PHP 5 and the integrated Zend Engine 2 have greatly improved PHP's performance and capabilities, but great care has been taken to break as little existing code as possible. So migrating your code from PHP 4 to 5 should be very easy. Most existing PHP 4 code should be ready to run without changes, but you should still know about the few differences and take care to test your code before switching versions in production environments.

Fortunately, it looks like the few incompatible changes that people will see have to do with string manipulation (something PHP is good at anyway). And, of course, there are all of the new functions that will come out of the move - check out their list here...