How to group by MySQL data, sum it up, and put into a PHP array

I am adding a chart to my application and all I want is to group MySQL data by date (month) and then to put it into an array for multiplication, but as I see MySQL groups only the first values of each month and I need the whole monthes grouped.

$query_purchase_grp = "SELECT item_qty, item_cost, date FROM purchase WHERE date BETWEEN '$date_1' AND '$date_2' AND $warehouse_data AND company = '$login_session_company' GROUP BY MONTH(date)";
$rs_purchase_grp = mysqli_query($con,$query_purchase_grp) or die("Error : ".mysqli_error());

while($data_purchase_grp = mysqli_fetch_assoc($rs_purchase_grp)){
    // Put mysql data into array for multiplication   
    $purchases_grp[] =  $data_purchase_grp['item_qty']*$data_purchase_grp['item_cost'];
}   
echo implode(",",$purchases_grp);