What is wrong with this update expiration and membership?
$unit = "Days";
$length = "1";
$newExpire = strtotime("+{$length} {$unit}");
$sqlz = "UPDATE users SET expire ='$newExpire' WHERE username ='{$_POST['custom']}'";
$sql = "UPDATE users SET membership ='14' WHERE username ='{$_POST['custom']}'";
$retval = mysql_query( $sqlz, $sql, $conn );
your mysql_query parameter is wrong, multiple queries are not supported so, change
$retval = mysql_query( $sqlz, $sql, $conn );
to
$retval = mysql_query($sql, $conn );
you need to run mysql_query for each query individually