Free Online Tutorials

First: creat func.php file

<?php
// tutorialspots.com
function get_rand_id($length)
{
  if($length>0)
  {
  $rand_id="";
   for($i=1; $i<=$length; $i++)
   {
   mt_srand((double)microtime() * 1000000);
   $num = mt_rand(1,36);
   $rand_id .= assign_rand_value($num);
   }
  }
    return $rand_id;
}

function assign_rand_value($num)
{
// accepts 1 - 36
  switch($num)
  {
    case "1":
     $rand_value = "a";
    break;
    case "2":
     $rand_value = "b";
    break;
    case "3":
     $rand_value = "c";
    break;
    case "4":
     $rand_value = "d";
    break;
    case "5":
     $rand_value = "e";
    break;
    case "6":
     $rand_value = "f";
    break;
    case "7":
     $rand_value = "g";
    break;
    case "8":
     $rand_value = "h";
    break;
    case "9":
     $rand_value = "i";
    break;
    case "10":
     $rand_value = "j";
    break;
    case "11":
     $rand_value = "k";
    break;
    case "12":
     $rand_value = "l";
    break;
    case "13":
     $rand_value = "m";
    break;
    case "14":
     $rand_value = "n";
    break;
    case "15":
     $rand_value = "o";
    break;
    case "16":
     $rand_value = "p";
    break;
    case "17":
     $rand_value = "q";
    break;
    case "18":
     $rand_value = "r";
    break;
    case "19":
     $rand_value = "s";
    break;
    case "20":
     $rand_value = "t";
    break;
    case "21":
     $rand_value = "u";
    break;
    case "22":
     $rand_value = "v";
    break;
    case "23":
     $rand_value = "w";
    break;
    case "24":
     $rand_value = "x";
    break;
    case "25":
     $rand_value = "y";
    break;
    case "26":
     $rand_value = "z";
    break;
    case "27":
     $rand_value = "0";
    break;
    case "28":
     $rand_value = "1";
    break;
    case "29":
     $rand_value = "2";
    break;
    case "30":
     $rand_value = "3";
    break;
    case "31":
     $rand_value = "4";
    break;
    case "32":
     $rand_value = "5";
    break;
    case "33":
     $rand_value = "6";
    break;
    case "34":
     $rand_value = "7";
    break;
    case "35":
     $rand_value = "8";
    break;
    case "36":
     $rand_value = "9";
    break;
  }
    return $rand_value;
}
?>

Usage:

<?php
include('func.php');
$length=15;
echo get_rand_id($length);
?>

Result:
dd2djhiwgdpgzn9
n5ktlbi4roi6zhy
….

Alternative method, very short:

<?php
/**
 * @author phptuts
 * @link http://tutorialspots.com/php-generate-random-ids-199.html
 */

function get_rand_id2($length, $chars = 'abcdefghijklmnopqrstuvwxyz0123456789')
{
    if ($length > 0) {
        $rand_id = "";
        for ($i = 1; $i <= $length; $i++) {
            mt_srand((double)microtime() * 1000000);
            $num = mt_rand(0, strlen($chars) - 1);
            $rand_id .= $chars{$num};
        }
    }
    return $rand_id;
}
?>