discount voucher script not working

i have script doesnt work ....(when doesnt work...nothing happens when add discount code , submit page)

in backend want able add own codes script either discounts set fee or percentage

for example

ds123%10

for %10

ds345£10

for £10

on shopping cart have following

mysql_select_db($database_lotties, $lotties);

$query_rsvoucher = "select * lottie_vouchercode";

$rsvoucher = mysql_query($query_rsvoucher, $lotties) or die(mysql_error());

$row_rsvoucher = mysql_fetch_assoc($rsvoucher);

$totalrows_rsvoucher = mysql_num_rows($rsvoucher);

// voucher code

if (isset($_post['vouchcode']) && $_post['vouchcode'] == $row_rsvoucher['vcode']) {

$mycode = $row_rsvoucher['vcode'];

$spos = strpos($mycode, "%");

if ($spos !== false) {

$myvalue = substr($mycode, $spos+1);

$myvalue = $xcart_sumtotal * $myvalue / 100;

} else {

$spos = strpos($mycode, "£");

if ($spos !== false) {

$myvalue = substr($mycode, $spos+1);

}

$mytotal = $xcart_sumtotal - $myvalue;

$_session['vouchcode'] = $myvalue;

} else unset($_session['vouchcode']);

then form

once form has been submitted echo shown

<?php echo doformatcurrency($myvalue, 2, ',', '.', '£ ', ''); ?>

if working should sent checkout....

thanks in advance

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