Here's a little snippet I got from SitePoint that I now use all the time. Add the code below to the start of each script.

Basically, what this does is checks to see if magic_quotes_gpc() is enabled on the server, and if it is, then it gets rid of all the slashes that magic_quotes_gpc() adds to input from $_GET, $_POST and $_COOKIES globals.

It's a good snippet to use, because it negates the bad programming practices that having magic_quotes_gpc() lets you get away with, and means that you don't really on PHP to validate your input; you get to do it all yourself ;-)

Creates a random string, usefull for passwords etc.

Use: $password = makeRandomPassword($Length);

If $Length is not specified (ie $password = makeRandomPassword();) then it will default to 8 characters

PHP function to validate an email address.

A little function that does some basic checking for data input by a user. Should get rid of code injection ;-)