To answer your question, the current method would be to create the object then call the method:
$functions = new Functions();
$var = $functions->filter($_GET['params']);
Another way would be to make the method static since the class has no private data to rely on:
public static function filter($data){
This can then be called like so:
$var = Functions::filter($_GET['params']);
Lastly, you do not need a class and can just have a file of functions which you include. So you remove the class Functions and the public in the method. This can then be called like you tried:
$var = filter($_GET['params']);