In this new post to the php|architect blog Matt Butcher offers a security tip for all PHP developers out there - never use $_GET again.
You don’t need to use $_GET or $_POST anymore. In fact, you probably shouldn’t use $_GET and $_POST anymore. Since PHP 5.2, there is a new and better way to safely retrieve user-submitted data. [...] Rather than accessing the $_GET and $_POST superglobals directly, you can make use of PHP functions like filter_input() and filter_input_array().
He gives a code example of it in use and talks about the two things these functions do to help keep you safe - validate the data for correct match on criteria and sanitizing the value to ensure the return value is only what's requested. You can find more about these filter functions in the Filters section of the PHP manual.