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$abc1 = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';
$zyx1 = strrev($abc1);
$abc2 = strtolower($abc1);
$zyx2 = strrev($abc2);

$text = strtr($text, $abc1.$abc2, $zyx1.$zyx2);

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<form name="form1" method="post" action="">
  <table width="314" border="0" cellspacing="0" cellpadding="0">
    <tr>
      <td width="145">Beschreibung des Item: </td>
      <td width="169"><textarea name="text" id="text"></textarea></td>
    </tr>
    <tr>
      <td>&nbsp;</td>
      <td><input type="submit" name="Submit" value="&Uuml;bersetzen">
      <input name="fertig" type="hidden" id="fertig" value="1"></td>
    </tr>
  </table>
</form>
<?php
if ($fertig){
$abc = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';
	$zyx = strrev($abc);

	$text = strtr($text, $abc, $zyx);

echo $text;
}
?>