In a recent post to his blog Tony Bibbs shares his experience with getting the most recent release (PHP 5.3 alpha) of PHP up and working on his MacBook.
If there is anything you should gleam from this article for future reference, Leopard comes with a 64bit Apache installation.
He points out this fact because, if you go and build it as per the normal install instructions, it will toss a "wrong architecture" error. He points out a blog post from Marc Liyanage that helped him understand a bit better. His fix was to run Apache as 32 bit instead of trying to get PHP to compile up to its 64 bit standards. He even includes the two commands you'll need to change Apache over to run this way.