Generating a unique random string of a certain length and restrictions in PHP?

function randr($j = 8){
    $string = "";
    for($i=0; $i < $j; $i++){
        $x = mt_rand(0, 2);
        switch($x){
            case 0: $string.= chr(mt_rand(97,122));break;
            case 1: $string.= chr(mt_rand(65,90));break;
            case 2: $string.= chr(mt_rand(48,57));break;
        }
    }
    return $string; 
}

echo randr(); // b53m1isM

function getRandomString($length = 8) {
    $characters = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
    $string = '';

    for ($i = 0; $i < $length; $i++) {
        $string .= $characters[mt_rand(0, strlen($characters) - 1)];
    }

    return $string;
}