How to loop and calculate to zero php

I have a array with values. I want to reduce the amount if someone buys an item.
For example if someone buys 45 items I want to loop the array values and calculate with the purchaded items until whats left.

What I tried.

 <?php
  $purchased = 45;
  $stock = array(40, 50, 60);

   for( $i = 0; $i < count($stock); $i++ ){
     $left = $stock[$i]-$purchased;
     echo $left . "\n";
   }

// result: -5, 5, 15
// needs to be: 0, 45, 60
?>

As suggested to show the code. its dutch column names. It has nothing to do with purchasing. I did it in the example to clarify.

$select = $db->prepare("SELECT c.id, d.id AS grondstofid, d.naam, 
(a.aantal/b.volume)*c.aantal AS afschrijving 
FROM ".DB04.".verkooporderregelstemp AS a 
INNER JOIN ".DB02.".recepten AS b ON a.receptid=b.id 
INNER JOIN ".DB02.".receptgrondstoffen AS c ON a.receptid=c.receptid
INNER JOIN ".DB02.".grondstoffen AS d ON c.grondstofid=d.id 
WHERE a.guid=:guid AND a.productie='1' AND c.grondstofid='1' GROUP BY c.id");
    $select->bindValue(":guid", $guid);
    $select->execute();
    $result = $select->fetchAll();
    foreach ($result as $data) {
      $grondstofid = $data['grondstofid'];
      $afschrijving = $data['afschrijving']; // this is the total reduction 

      $select1 = $db->prepare("SELECT id, voorraad FROM ".DB02.".grondstofbatch WHERE grondstofid=:grondstofid AND FROM_UNIXTIME(thtdatum)>CURRENT_TIMESTAMP GROUP BY id ORDER BY thtdatum ASC");
      $subselect->bindValue(":grondstofid", $grondstofid);
      $subselect->execute();
      $subresult = $subselect->fetchAll();
      foreach ($subresult as $subdata) {
       $subdata['voorraad']; // 3 rows
       
       //batch 1 - AMOUNT 40
       //batch 2 - AMOUNT 50
       //batch 3 - AMOUNT 60

       // some loop for $subdata['voorraad']-$afschrijving
       // update SQL
       // if its hits zero get remaining $afschrijving goto next row
      }
    }

>Solution :

You can achive that with a simple foreach loop.

$purchased = 45;
$stock     = [40, 50, 60];

$newStock = function ($purchased, $stock) {
    $items = [];
    foreach ($stock as $item) {
        $delta     = $item - $purchased;
        $items[]   = $delta > 0 ? $delta : 0;
        $purchased = $delta < 0 ? abs($delta) : 0;
    }
    return $items;
};

print_r($newStock($purchased, $stock));

Output

Array
(
    [0] => 0
    [1] => 45
    [2] => 60
)