<?php 
print "<form name=\"test\">
         <input type=\"hidden\" name=\"wert\" value=\"$_POST[whatever]\">
         </form>
<script type=\"text/javascript\">

 new Option(document.test.wert.value,document.test.wert.value,false,true);
 document.anderes.listbox.options[document.anderes.listbox.length];
</script>
<form name=\"anderes\" action=\"whatever.php\" method=\"post\">
<select name=\"listbox\" size=\"30\" multiple>
</select>
</form>