Some help

I had an idea of using the switch() function in PHP to switch the arguments inside of a function. Only thing is, it doesn't really work, and I am not sure why.

function VALIDATE_($value)
        {
        switch ($value) {
            case $user:
                return preg_match('/^[A-Z0-9]{2,20}$/i', $username);
                break;
            case $mail:
                return filter_var($email, FILTER_VALIDATE_EMAIL);
                break;
            }
        }

I am sure you can see what I am trying to do. I want it to work so that whenever I change $value to $user or $email the output is different depending on which I use.

But whenever I use VALIDATE_($mail);, it always uses the first case. Anyone have an idea on how I can fix this?