How to generate a unique code in laravel?

I am using this function You may use it like something like bellow

$code = $this->generateRandomString(6);// it should be dynamic and unique 

public  function generateRandomString($length = 20) {
        $characters = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
        $charactersLength = strlen($characters);
        $randomString = '';
        for ($i = 0; $i < $length; $i++) {
            $randomString .= $characters[rand(0, $charactersLength - 1)];
        }
        return $randomString;
    }