I tried with this little script (vars.php):

<?php echo $LANG;?>
<form action="vars.php" method="post">
<input type="text" name="LANG">
<input type="submit">
</form>

and it works like it should. ie. When loaded without posting,
the LANG is en_US, and when I post that form, it's set whatever
I type in the input field. Is this what you meant here?

--Jani

This must have been an error in my script. The variable was not overwritten, because in some cases the script did not assign it a value. Thank you and sorry.