Get unique string in length

unique-string.php

<?php
function($length) {
  $char = "abcdefghijklmnopqrstuvwxyz0123456789";
  $char = str_shuffle($char);
  for($i = 0, $rand = '', $l = strlen($char) - 1; $i < $length; $i ++) {
      $rand .= $char{mt_rand(0, $l)};
  }

  return $rand;
};