Rand String Generator - Buggy

<?php

function genRandomString($length) {
   $characters = '0123456789ABCDEFGHIJKLMNOPQRSTUXWXYZabcdefghijklmnopqrstuvwxyz';
   $string = "";

   for ($p = 0; $p < $length; $p++) {
       $string .= $characters[mt_rand(0, strlen($characters))];
   }

   return $string;
}

$i = 0;

$total = 100;

while($i < $total)
{
    $code = genRandomString(15);
    $length = strlen($code);

    if($length == 15)
    {
        echo "<span style=\"background-color: green;\">$code - $length chars</span><br/>";
    }
    else
    {
        echo "<span style=\"background-color: red;\">$code - $length chars</span><br/>";
    }

    $i++;
}


?>