PHP：每个之后丢失的数组值

first let me explain what I am trying to achieve:

I've got an array of user items, consisting of an ID(item_id) and quantity(e.g. 10 items) If a user purchases an item, it's added to the array including the quantity. If a user purchases an (in the array) existing item, '1' is added to the quantity.

I got really close with the help of this post: Checking if array value exists in a PHP multidimensional array

this is the code I am using right now:

 $item_id = arg(1);
  $quantity = '1';

  $found = false;
  $bought_items = null;
  $data = null;

  foreach ($user_items as $key => $data) {

    if ($data['a'] == $item_id) {
        // The item has been found => add the new points to the existing ones
        $data['b'] += 1;
        $found = true;
        break; // no need to loop anymore, as we have found the item => exit the loop
    } 
  }

  if ($found === false) {

      $bought_items = array('a' => $item_id, 'b' => $quantity);
  }

  $array = array($bought_items, $data);

If the item_id is non existing, it is added to the array If the item_id is existing, the quantity will 'receive' +1

so far so good

now the actual problem, let's sketch the scenario:

I purchase item 500 -> array contains: id=500, quantity=1

I purchase item 500 -> array contains: id=500, quantity=2

I purchase item 600 -> array contains: id=500, quantity=2, id=600, quantity=1

after this it goes wrong

I then purchase item 500 or 600, the other item is removed from the array. So when I purchase item 500, item 600 and its quantities are removed from the array.

I've been puzzling for hours but can't find the mistake, I know I'm overlooking something logical. I think it's going wrong in the for each.

I tried for example this, and it works, so u can change to own use. The code of another post is useless for your purpose

    $item_id = 500;
    $quantity = 1;

    $user_items = array(400, 300, 200, 500, 500, 200, 500, 500);
    $found = FALSE;
    $bought_items = null;
    $data = null;

    foreach ($user_items as $data) {

        if ($data == $item_id) {
            // The item has been found => add the new points to the existing ones
            $quantity += 1;
            $bought_items[$data]['a'] = $data;
            $bought_items[$data]['b'] = $quantity;
            $found = TRUE;
        }

        if ($found === FALSE) {

            $bought_items[$data] = array('a' => $data, 'b' => $quantity);
        }
        $found = FALSE;
    }
    print_r($bought_items);

Output:

array(4) {
   400 => array(2) {
      a => 400
      b => 1
   }
   300 => array(2) {
      a => 300
      b => 1
   }
   200 => array(2) {
      a => 200
      b => 3
   }
   500 => array(2) {
      a => 500
      b => 5
   }
}

If bought_items is an array then you're overriding your values rather then adding them to the array.

$bought_items = array('a' => $item_id, 'b' => $quantity);

should be:

$bought_items[] = array('a' => $item_id, 'b' => $quantity);