insert เข้า database ไม่ได้ echo ออกปกติ แต่ไม่เข้าไปเก็บใน sql

include 'config.php';				
	$q = "select * from product_cart where member_id = '".$_SESSION["num_sum"]."'";
	$w = mysql_query($q);				  	  	  
	while ($z = mysql_fetch_array($w))
{
	$cart = $z['pcart_name'];
	$cart2 = $z['pcart_qty'];
	$cart3 = $z['pcart_price'];			
												
	$a = "INSERT INTO order ";
	$a .="(order_bill,order_date,member_id,pcart_name,pcart_qty,pcart_price,order_payment,order_status) ";
	$a .="VALUES ";
	$a .="('".$_SESSION['bill']."','".$_SESSION['date']."','".$_SESSION['id']."','".$cart."','".$cart2."','".$cart3."','".$_POST['select']."',1) ";				
	      
        $b = mysql_query($a);
}