Question

please why is the loop statement breaking, after the multiplication it only update the last row, tried using a foreach loop for the $totalprice in the update statement it says invalid argument.

if(array_key_exists('item', $_POST)){


// $items = $_POST['item'];
  //foreach($_POST['item'] as $item){
    //echo $item['Pquantity'] . ", ";
    //echo $item['Pidno'] . ", ";
   // }
    //Loop through $_POST items, updating the database for each item
    foreach ($_POST['item'] as $item) { 
        $Pquantity = intval($item['Pquantity']);
        $Pidno = ($item['Pidno']); 
        //echo $Pquantity . ", ";
        //echo $Pidno . ", ";
        //$totalprice = intval($item['Pquantity']) 
        $queryreg = mysql_query("
            UPDATE repplac
                 SET Pquantity = {$Pquantity}
                 WHERE
                       Pidno = '{$Pidno}'
                 AND
                       Uname = '{$_SESSION['username']}'
        ") or die(mysql_error());  

    }
    }
    $pplresult = mysql_query("SELECT * FROM repplac WHERE Uname = '{$_SESSION['username']}'") or die(mysql_error());
  while ($row = mysql_fetch_assoc($pplresult)) 
  {
  $totalprice = $row['Price'] * $row['Pquantity'];
  //echo "$totalprice";
  //die();
   $queryreg = mysql_query("
  UPDATE repplac
  SET Tprice = {$totalprice}
  WHERE
                       Pidno = '{$Pidno}'
                 AND
                       Uname = '{$_SESSION['username']}'
    ") or die(mysql_error());  
  }

Answer 1

Inside your foreach you update $Pidno with the following code

$Pidno = ($item['Pidno']);

Thus you are updating different records in the database. But inside your while loop $Pidno is staying the same the entire time. Thus you are updating the same record over and over. To fix this add the following code at the beginning of the while loop

while ($row = mysql_fetch_assoc($pplresult)) 
{
    //get the correct Pidno for this row
    $Pidno = ($row['Pidno']); 
    $totalprice = $row['Price'] * $row['Pquantity'];

    // ... the rest of your while loop