PHP 4 and the integrated Zend engine 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 3 to 4 should be
much easier than migrating from PHP/FI 2 to PHP 3. 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. The following should give you some
hints about what to look for.