Cute trick under linux:
Make sure you have binfmt_misc enabled
Install PHP(5 if you want)
Then do something like this:
echo ":PHP:E::php5::/usr/local/bin/php5:" > /proc/sys/fs/binfmt_misc/register
Then enable regular CGI in apache and make sure .php or .php5 is counted as CGI type. It'll get executed as a CGI yes, but it'll operate perfectly with any apache mods or even another version of PHP. I use it on my hosting service to provide both PHP4 and PHP5 at the same time.
Also recommend looking at PHP5. Its a bit beta at the moment but proper object semantics and exceptions are well worth while.