.php, .php3, .php4
 

Whats the diffrence?

Nothing. It's just the extension. Most servers allow all the extensions listed above, but some just allow .php. It's a server choice. Most scripts using .php3 are old. The norm. is .php but you can use anything you want to, if you script the server right, using a .htaccess file.

If your server runs on PHP3, there is a BIG difference though. PHP3 is so old that it doesn't have a great many functions that you have in PHP4, as well as tons of other things that will require your scripts to be quite different to support PHP3...