Filtering user input is important.
Server-side validation is important as it truly validates the user's input before it is processed. Implementing a client-side validation only is insufficient as it makes the assumption that the only users on your website are normal users. Oftentimes there are bots or even malicious attacker that can inject codes through vulnerable inputs. This is why you should always have server-side validation in your code. All server variables such as $_GET, $_POST, $_COOKIE, and even $_SERVER still need to be validated as it can be changed.
SCHLIX CMS provides the following functions to validate $_GET and $_POST to filter and convert user input to the desired input type: all functions that start with fget_ and fpost_.