Fabien Potencier has been moving over to the Mac OSX platform for some of his development and figured out that he needed a custom installation of PHP 5.3, different than the one that comes with the default install of Snow Leopard. The process was a bit tricky, so he's written up a post as a guide for those wanting to do it too.

To do my work, I need to have several different versions of PHP at hand, with different configurations, and so I always compile my own PHP and Apache binaries on my Mac. [...] Now that I use PHP 5.3.0 more and more, I decided it was time to compile my own version. As it turns out, compiling PHP 5.3.0 on Snow Leopard is a bit involving.

He walks you through the configure command he used, how he patched the iconv.c file, patch the source of PHP with the change and recompile with an EXTRA_LIBs environment variable set. The steps he followed were based on the hints provided in this bug from the PHP.net website.