I have recently updated to Lion and enabled Web Sharing in the System Preferences but I am unable to get php working.
I added an info file to the web root directory and it outputs the file as text.
info.php
with the content
<?php phpinfo(); ?>
(Edit: This method appears to work fine for 10.9 (Mavericks), 10.10 (Yosemite) and 10.11 (El Capitan), so I figured I'd mention that for any new influx of slightly frustrated OS X updaters :D )
Edit your /etc/apache2/httpd.conf and make sure the line:
LoadModule php5_module libexec/apache2/libphp5.so
...exists. I think it's commented out by default in the standard OS X config, but from what I remember, you just need to uncomment it, then re-start Apache:
sudo apachectl restart
And you should be good to go.