however, since PHP also stores variables for system settings in this same global namespace, some of which are not always populated, it is pretty easy for a user to spoof your script by passing in variables with the correct names.
I wondered about this a few months ago, it seems like a pretty insecure system if you ask me. Wouldn't it also be possible for someone to use serious memory on the box by making a request like:

foo.php?foo=bar&baz=foo&bar=foo&morefoo=foo etc. I suppose it wouldn't be so much of a problem with GET, as surely it is limited by the size of an environment variable (at least on UNIX that is). What about POST though?

IMHO PHP may be ok for web/CGI-type apps, but you can't make a nice runnable "normal" program with it, can you? Therefore I would imagine that learning Perl is a better use of your time.