improve php function random string

I have function, wich generates random string, its ok but I want to generate unique numbers, this function returns generated nunbers again.

function getRandomString($length){
$validCharacters = "12345678910";
$validCharNumber = strlen($validCharacters);
$result = "";
    for($i = 0; $i < $length; $i++){
        $index = mt_rand(0,$validCharNumber - 1);
        $result .= $validCharacters[$index];
    }
return $result;}

$x= getRandomString(2).substr(time(),-4);//rand(1000000,9999999);