Listing 5. Checking Out and Reducing Inventory

<?php


function cart_checkout($credit_card,$address,$name) {
global $conn, $customer_id, $feedback;

//start a transaction
query("BEGIN WORK");

# lock the appropriate rows in the item_inventory
# table, based on what is in the visitor's
# cart. We'll do this with a simple
# subselect
*/
$sql="SELECT * FROM item_inventory ".
     "WHERE part_number ".
     "IN (SELECT part_number FROM cart_items ".
     "WHERE customer_id='$customer_id') ".
     "FOR UPDATE";
$res=query($sql);

//check for errors
if (!$res || pg_numrows($res)<1) {
      //no items matched or db failed
      $feedback .= pg_errormessage($conn);
      $feedback .= ' Error - no items locked ';

      //terminate the transaction
      query("END WORK");
      return false;
} else {
      /*
# Inventory rows are now locked
# Get the items and quantity from the cart

  
      $sql="SELECT part_number,quantity ".
              "FROM cart_items ".
              "WHERE
customer_id='$customer_id' ".
              "ORDER BY part_number DESC";
       $res2=query($sql);

       //check for errors
       if (!$res2 || pg_numrows($res2)<1) {
       //no items selected from cart
               $feedback .= pg_errormessage($conn);
               $feedback .= ' Error - no items in cart ';

               //terminate the transaction
               query("END WORK");
               return false;
        } else {
               $rows=pg_numrows($res2);
               /*
# Inventory is locked and we have the cart contents
# Iterate and update the inventory balances

                         for ($i=0; $i < $rows; $i++) {
                         //fetch this row from our cart
                         $quantity=pg_result($res2,$i,'quantity');
                         $item_id=pg_result($res2,$i,'part_number');

                         $res3=query("UPDATE item_inventory".
                         "SET inventory =inventory-$quantity ".
                         "WHERE part_number='$item_id'");

                         //see if query failed or no rows were affected
                         if (!$res3 || pg_cmdtuples($res3) < 1) {
                         //couldn't update this row
                         $feedback .= pg_errormessage($conn);
                         $feedback .= ' Error - updating inventory failed ';

                         //rollback the transaction
                         query("ROLLBACK");
                         return false;
                         }
                 }

                 /*
# Inventory now fully updated
# Finally - let's get the total amount
# of this order and update the customer record
                 */
                 $res=query("SELECT sum(cart_items.quantity*item_inventory.price) ".
                         "FROM cart_items,item_inventory ".
                         "WHERE cart_items.customer_id='$customer_id' ".
                         "AND cart_items.part_number=item_inventory.part_number");

                  //check for errors
                  if (!$res || pg_numrows($res) < 1) {
                         //couldn't get order total
                         $feedback .= pg_errormessage($conn);
                         $feedback .= ' Error - couldn\'t get order total ';

                         //rollback the transaction
                         query("ROLLBACK");
                         return false;
                  } else {
                         /*
# We've got the order total, now 
# we just update the customers table
                         */

                         //set the sum as a local variable
                         $total=pg_result($res,0,0);
                         $res=query("UPDATE customers ".
                                    "SET address='$address',name='$name',".
                                    "total_order='$total',credit_card='$credit_card'
".
                                    "WHERE customer_id='$customer_id'");

                          //the usual error check
                                     if (!$res || pg_cmdtuples($res) < 1) {

                                     //update failed or did not affect any rows
                                     $feedback .= pg_errormessage($conn);
                                     $feedback .= ' Error - updating customer information ';

                                     //rollback the transaction
                                     query("ROLLBACK");
                                     return false;
                          } else {
                                     /*
# We made it! 
# Now commit the changes to the database
                                     */

                                     //commit all changes
                                     query("COMMIT");

                                     //erase the PHP4 session
                                     $customer_id=0;
                                     session_destroy();
                                     return true;
                           }
                  }
           }
     }
}

?>