Migrating from PHP 4 to PHP 5.0.x
Backward Incompatible Changes
Migrating from PHP 4 to PHP 5.0.x
PHP Manual

## What has changed in PHP 5.0.x

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.

Migrating from PHP 4 to PHP 5.0.x
Backward Incompatible Changes
Migrating from PHP 4 to PHP 5.0.x
PHP Manual