The php.ini file is one of those bits of technological nonsense where if you have to ask then you probably don't wanna know  ;)

Seriously, I'd advise you to leave it alone at a time when (according to your other post) strange things are happening on the server – you could muddy the waters a little.

And the direct answer to your question is "it varies" and some web hosts won't let you anywhere near it.

If you're still determined to increase your upload limit then I suggest framing a precise question to your server support people on the lines of: "how can I change the two php.ini directives: post_max_size and upload_max_filesize."

jack