I have been using Mac OS X as my primary operating system for a few years now, and only today I have found a very neat way to debug PHP code, like it is common for application code (i.e. stepping through code for debugging purposes).
I am using the PHP package by Marc Liyanage almost ever since I have been working on OS X, because it's far more flexible than the PHP shipped with OS X.
Unfortunately, installing Xdebug the usual pecl install xdebug doesn't work. But on the internetz you can find a solution to this problem.
Basically you need to download the source tarball and use the magic command CFLAGS='-arch x86_64' ./configure --enable-xdebug for configuring it. (The same works for installing APC by the way)
/usr/local/php5/php.d $ cat 50-extension-xdebug.ini
Now you can use MacGDBp. There is an article on Particletree that describes the interface in a little more detail.
I really enjoy using this method to only fire up this external program, when I want to debug some PHP code, and can continue to use my small editor, so that I don't have to switch to a huge IDE to accomplish the same.