Mac OSX 10.4 still only ships with PHP4, which is fine and all, but I eventually bit the bullet and decided to install PHP5.

The most immediately obvious way to do this is with Marc Liyanage’s excellent installer. I unzipped, installed the .pkg, and rebooted Apache.

Except Apache didn’t want to reboot. Apache refused to start, actually. Looked like a potential crisis!

Fortunately, a few minutes of digging found the solution. Pretty obvious, really:

you can’t load both mod_php4 and mod_php5 Apache modules at the same time.

I had mod_php4 enabled already. By commenting out the lines referring to it, Apache started up just fine, running PHP 5. Crisis averted.