ด่วน!!! ทำโปรเจควัสดุครุภัณฑ์ แต่หน้าทำรายการสั่งซื้อครุภัณฑ์ กดเพิ่มลงรายการไม่ได้ ต้องทำอย่างไร

if($action=="add"){
    $dates=$_GET["dates"];
    $d_id=$_GET["d_id"];
    $num=$_GET["num"];
    $a=0;
    include "connect.php";
    $tb="buy_detail_durable";
    $sql="select * from $tb where username='$login_username' and status='0' and d_id='$d_id'";
    $result = mysql_query($sql);
    While($row= mysql_fetch_array($result)){
            $id_durable= $row["id_durable"];
            $nums= $row["num"];
            $a=1;
     }

    $num=$nums+$num;
    if($a==1){    
           $tb="buy_detail_durable";
           $add = mysql_query("UPDATE `$db`.`$tb` SET `num` = '$num' WHERE `id_durable` = '$id_durable' ");
    }
    if($a==0){
           $tb="buy_detail_durable";
           $sql="select * from $tb";
           $db_query=mysql_db_query($db,$sql);
           $num_rows=mysql_num_rows($db_query);
           mysql_query("INSERT INTO $tb (d_id,num,username,status) values('$d_id','$num','$login_username','0')") 
           or die ("<center>ไม่สามารถ Upload ได้</center> ");

      }
      echo "<META HTTP-EQUIV=refresh CONTENT=\"0 ; URL=admin_buy_durable.php?action=make&dates=$dates\">";
}