PHP Manual . Prev . Next . ... Running PHP 3 and PHP 4 concurrently . Migrating Configuration Files . ... PHP 4 and the integrated Zend engine have greatly improved PHPs performance and capabilities, but great care has been taken to break as little existing code as possible. ... A lot of existing PHP 3 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. ...