Apache and PHP are installed by default in Tiger (and previous versions of Mac OS X).
When you upgrade to Tiger, it overwrites your apache config file (/private/etc/httpd/httpd.conf), and the version that is installed has PHP disabled. Edit that file and uncomment the LoadModule and AddModule directives for mod_php. You can find these easily enough by searchig the file for “php”.
It is documented that this file may get replaced with OS upgrades, so the best way to make changes to your apache config is to add directives to a file in /private/etc/httpd/users. Files in that directory do not get overwritten, and get loaded by the main apache config file. So if you want PHP to stay active next upgrade, you could add the LoadModule and AddModule directives in there, rather than uncomment them in the main config file.
