How can I get a unique value for a VARCHAR field for usernames?

I have a login-process where I would like to prefill as much as possible. The username has to be unique, so I have to get a unique, "random" string. How do I do that?

I'm currently thinking about something like this:

function getRandomString($length = 5) {
    $characters = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
    $string     = ''; 
    for ($i = 0; $i < $length; $i++) {
        $string .= $characters[rand(0, strlen($characters)-1)];
    }
    return $string;
}

function getUniqueVarchar($prefered) {
    // Try all combinations that would make sense to the user. Eventually give an array and try to combine elements like first name / last name / birthday
    $usernameTry = $prefered
    $exists = Does the username $usernameTry exist?
    if (!$exists) return $usernameTry;

    // Maybe a year exists?
    for($i=50;$i<99;$i++) {
        // $exists = Does the username $usernameTry = $prefered.$i exist?
        if (!$exists) return $usernameTry;
    }

    // Try random strings
    for($i=0;$i<10;$i++) {
        // $exists = Does the username $use开发者_开发知识库rnameTry = getRandomString() exist?
        if (!$exists) return $usernameTry;
    }
    return false;
}

Does something better than my part at "// Try random strings" exist? Does MySQL (or other DB systems) have something like that built-in?