URL Friendly Username in PHP?

On my PHP site, currently users login with an email address and a password. I would like to add a username as well, this username they g\set will be unique and they cannot change it. I am wondering how I can make this name have no spaces in it and work in a URL so I can use there username to link to there profiles and other stuff. If there is a space in there username then it should add an underscore jason_davis. I am not sure the best way to do this?

function Slug($string)
{
    return strtolower(trim(preg_replace('~[^0-9a-z]+~i', '-', html_entity_decode(preg_replace('~&([a-z]{1,2})(?:acute|cedil|circ|grave|lig|orn|ring|slash|th|tilde|uml);~i', '$1', htmlentities($string, ENT_QUOTES, 'UTF-8')), ENT_QUOTES, 'UTF-8')), '-'));
}

$user = 'Alix Axel';
echo Slug($user); // alix-axel

$user = 'Álix Ãxel';
echo Slug($user); // alix-axel

$user = 'Álix----_Ãxel!?!?';
echo Slug($user); // alix-axel

In other words... you need to create a username slug. Doctrine (ORM for PHP) has a nice function to do it. Doctrine_Inflector::urlize()

EDIT: You should also keep username slug in database, as a Unique Key column. Then every search operation should be done based on that column, not original username.