lose the last comma off a list

rtrim.php

<?php

$sql = "UPDATE coupons SET ";
foreach($args as $key => $arg) {
	$arg = $this->db->escape($arg);
	$key = $this->db->escape($key);
	$sql .= "$key = $arg, ";
}
//lose the last comma
$sql = rtrim($sql, ", ");

$sql .= " WHERE coupon_code = $coupon_code";