สวัสดีครับรบกวนขอความรู้เรื่อง การเอาค่าที่ได้จากตาราง มา Sum กันอ่าครับ

$dateStart = "";
$dateEnd = "";
$bookingReportResult = array();
if($_POST){
  $whereClause = 'WHERE f_user = "'.$_SESSION['MM_Username'].'" AND f_password = "'.$_SESSION['field_pass'].'" AND court.court_id = '.$_POST['court'];
  if($_POST['dateStart']){
    $dateStart = $_POST['dateStart'];
    $whereClause .= ' AND booking.booking_date >= "'.$_POST['dateStart'].'"';
  }
  if($_POST['dateEnd']){
    $dateEnd = $_POST['dateEnd'];
    $whereClause .= ' AND booking.booking_date <= "'.$_POST['dateEnd'].'"';
  }
   $bookingHisSql = '
    SELECT
      *,
      CONCAT(cust.c_name, " ", cust.c_last) AS booking_name
    FROM 
      booking AS booking
    INNER JOIN
      field AS field ON field.field_ID = booking.ref_field_id
    INNER JOIN
      court AS court ON court.court_id = booking.court_num AND court.ref_field_ID = booking.ref_field_id
    INNER JOIN
      customer AS cust ON cust.customer_ID = booking.ref_customer_id
      '.$whereClause.'
    AND booking.status="1" ORDER BY booking.booking_date ASC
    
  ';
  $bookingHisQry = mysql_query($bookingHisSql);
  $bookingReportResult = array();
  while(($res = mysql_fetch_assoc($bookingHisQry))) {
    $bookingReportResult[] = $res;
  }
}

foreach($bookingReportResult as $key => $booking) {
        echo '
          <tr>
            <td><center>'.$booking['booking_id'].'</center></td>
            <td>'.$booking['booking_date'].'</td>
            <td>'.$booking['booking_time'].'</td>
            <td><center>'.$booking['court_num'].'</center></td>
            <td><center>'.$booking['cost'].'</center></td>
            <td><center>'.$booking['booking_name'].'</center></td>
          </tr>
        ';
      }