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.
