Username of First name and first letter Last name

Is it possible to assign a username to a new user using the First Name + 1st letter of the Surname? Like Robert R., I think I'm in the right direction using http://php.net/manual/en/function.substr.php
but can't get it work. Any suggestions on this are welcome!

<?php
       add_filter('gform_username', 'auto_username');
function auto_username($username){

    $username = strtolower(rgpost('input_1_3') . $string[0];(rgpost('input_1_4')));

    if(empty($username))
        return $username;

    if(!function_exists('username_exists'))
        require_once(ABSPATH . WPINC . "/registration.php");

    if(username_exists($username)){
        $i = 2;
        while(username_exists($username . $i)){
            $i++;
        }
        $username = $username . $i;
    };

    return $username;
}
?>