While waiting for Marc from entropy for “official” easily installable PHP packages for OS X 10.5 aka Leopard, Liip's local.ch master Tobi managed to do it by himself (What's a Liip developer without a working PHP and some not so common extension? :) )
Read by yourself over at his blog how he did it and what's do to to get it running, without having to recompile Apache as well.