For me, in my fresh Snow Leopard install, PHP5 wasn’t enabled by default.  Here is what I had to do to get it working:

In the /etc/apache2/httpd.conf file, underneath all the “LoadModule” commands I added:

LoadModule php5_module libexec/apache2/libphp5.so

Then in terminal I ran the command:

sudo apachectl -k restart

And PHP5 was working for me. Feel free to post any questions in the comments. Thanks!