PHP - How to make a random unique string

makeUniqueString.php

<?PHP
function makeUniqueString ($length=16) // $lenght - string lenght
{
           $salt       = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ012345678';
           $len        = strlen($salt);
           $makepass   = '';
           mt_srand(10000000*(double)microtime());
           for ($i = 0; $i < $length; $i++) {
               $makepass .= $salt[mt_rand(0,$len - 1)];
           }
       	   return $makepass;
}

// echo
echo makeUniqueString();
?>