I have been using FreeBSD for server.
Configuring PHP is not as simple as compiling or installing a package. There are too much issues to take care, such as Apache 1 vs Apache 2, which modules to be installed, and whether security features to be burned in. It is getting less and less possible to have a one-configuration-fit-all binary file.
The inflexibility of configuring PHP has been my major obstacle moving to Mac OS X. Definitely OS X's interface is superior. However there is no guarantee that all the modules I want will be kept in the binary updates. A few years ago, PHP GD module (which modifies photo) was in OS X. After a specific update of 10.2, it is gone forever.
I would choose a system which has extensive port support (a service that automatically downloads, configures and compiles software). FreeBSD is a perfect choice in this aspect. You can choose to compile for a specific version of Apache, and have total freedom in choosing what modules to be installed. Yes it do take you some time. However it also gives you the flexibility to optimize and peaceful mind. When something can be compiled successfully on your computer, very likely it can actually run, too.
I know some friends using Fedora. They were just mad (angry) about installing a specific PHP module. Somehow Fedora didn't happen to have that package available. FreeBSD, on the other hand, have most of the modules in source code, and precompiled for admin's choice. If you do not believe in my claim, you can have a search of "php" in both systems' online package search. You will notice a huge difference.
If you choose to use Linux, just like what others have suggested above, try Debian. It's "port system" (I forgot their name for that counterpart) is very advanced.