One word: FINK
PHP is installed by default with OS X 10.1.# but is NOT executed by Apache by default - you will need to edit your APACHE.CONFIG file...unless you use FINK, which will install the most recient version. HOWEVER, to use FINK, you will need a working XWindows server (XFree)...
In other words, if you don't know what you're doing, I wouldn't start messing with it...