PHP has already a long history behind him : Legendary
PHP 1.0, PHP/FI, PHP 3.0 and PHP 4.0.
PHP 4 was designed to be as compatible with earlier
versions of PHP as possible and very little
functionality was broken in the process. If you're
really unsure about compatibility you should install
PHP 4 in a test environment and run your scripts
there.
Also see the appropriate
migration appendix of this manual.