Once again, aye; check the server logs for details.
I also run Apache on FBSD, and the times I remember seeing this were usually due to the fact that the cgi-bin directory was not configured to run PHP scripts, IIRC; it's been quite a long time though, and I can't remember the details.
Some other possibilities include: improper script alias directory, ExecCGI not turned on for your cgi-bin directory, no AddHandler routine for *.php in the cgi-bin directory, etc. It's a webserver administrative issue, most likely.
If you are running your own server, you've a bit of research to do. If you're hosted, they should have tech support help you; if not, they're not worth what you are paying them, IMHO.
Good luck.