The filter functions are part of the PECL library and should come as standard on most PHP 5 installs. If they aren't there then ask your server administrator to install them.
The filter functions where created to avoid developers having to write lots of unmaintainable code in order to check the validity of variables and to sanitize these variables once validated. So rather than using many different functions and regular expressions to tell if a value is a number, a boolean or even a URL, you can just use these filter fucntions.
The main functions that you might be interested in are filter_var() and filter_input(). The filter_var() function is used to validate a single input, the parameters are: