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.