To turn on PHP, edit your Apace config file in /etc/httpd/httpd.conf using a text editor. (BBEdit is good, you can type bbedit /etc/httpd/httpd.conf at the command line to edit the file). Uncomment (remove the #) at any lines that mention PHP, should just be two in the default config file. Restart your websharing.
I don't know if Apple's PHP includes any extra extentions that the other PHP packages do.