Trying to create a scrambling script but got into a problem...

[Lock Down]

It's happening because it changes multiple times during the preg_replace.
try this:

<?php
$key = $_SERVER['QUERY_STRING'];
// used for testing
// $key = 'abcdefghijklmnopqrstuvwxyz0123456789';
$characters = array('/a/','/b/','/c/','/d/','/e/','/f/','/g/','/h/','/i/','/j/','/k/','/l/','/m/','/n/','/o/','/p/','/q/','/r/','/s/','/t/','/u/','/v/','/w/','/x/','/y/','/z/','/0/','/1/','/2/','/3/','/4/','/5/','/6/','/7/','/8/','/9/');
$scrambles = array('0','1','2','3','4','5','6','7','8','9','a','s','d','f','g','h','j','k','l','q','w','e','r','t','y','u','i','o','p','z','x','c','v','b','n','m');
$result = '';
$ctr =0;
$one = 1;
while ($ctr < strlen($key)) {
    $k = substr($key,$ctr,1);
    $out = preg_replace($characters[$ctr], $scrambles[$ctr], $k, $one);
  $result .= $out;
$ctr++; }
echo "Original:<br/>";
echo $key."<br/>";
echo "Scrambled:<br/>";
echo $result;
?>