Fout in MySQL

<?
$sql = "SELECT ISNULL (MAX (positienummer), 0) AS 'MAX' FROM winkelwagenregels WHERE winkelwagennummer = '$_SESSION[sess_winkelwagennr]' ";
?>

<?
$sql = "SELECT ISNULL (MAX (positienummer), 0) AS 'MAX' FROM winkelwagenregels WHERE winkelwagennummer = '$_SESSION[sess_winkelwagennr]' ";
?>

<?php
if ($_POST['artikelen'])
{
   $query = "SELECT * FROM artikelen WHERE levprodcode = '$_POST[levprodcode]' ";
   $result = mysql_query($query) or die(mysql_error());
   echo"<table> lev: {$_POST[levprodcode]}</table>";
   
   while ($show = mysql_fetch_array($result))
   {
       $pr = number_format(roundprijs(prijsinc(100,$show['verkoopex'],$show['verkoopminex'],$show['btwtarief'])),2,".","");

       $levernr = $show['leveranciernummer'];
       $levprod = $show['levprodcode'];
       $vendor = $show['vendorpartnr'];
       $omsch = $show['omschrijving'];
       $merk = $show['merk'];

       $sql = "SELECT ISNULL (MAX (positienummer), 0) AS 'MAX' FROM winkelwagenregels WHERE winkelwagennummer = '$_SESSION[sess_winkelwagennr]' ";
       $res = mysql_query($sql) or die(mysql_error());

       while ($shown = mysql_fetch_array($res))
       {
           $positie = $shown['positienummer'];
           echo"pos: $positie";

       }

//       $sql = "INSERT INTO winkelwagenregels (winkelwagennummer, positienummer, merk, omschrijving, vendorpartnr, apn, leveranciernr, aantal, stuksprijs) VALUES
//                                             ('$wagennr', '$aantal', '$merk', '$omsch', '$vendor', '$levprod', '$levernr', '$val', '$pr')";
//       $resultaat = mysql_query($sql) or die (mysql_error());

   }
}
?>

<?php
if ($_POST['artikelen'])
{
   $query = "SELECT * FROM artikelen WHERE levprodcode = '$_POST[levprodcode]' ";
   $result = mysql_query($query) or die(mysql_error());
   echo"<table> lev: {$_POST[levprodcode]}</table>";
 
   while ($show = mysql_fetch_array($result))
   {
       $pr = number_format(roundprijs(prijsinc(100,$show['verkoopex'],$show['verkoopminex'],$show['btwtarief'])),2,".","");
 
       $levernr = $show['leveranciernummer'];
       $levprod = $show['levprodcode'];
       $vendor = $show['vendorpartnr'];
       $omsch = $show['omschrijving'];
       $merk = $show['merk'];
 
       $sql = "SELECT ISNULL (MAX (positienummer), 0) AS 'MAX' FROM winkelwagenregels WHERE winkelwagennummer = '$_SESSION[sess_winkelwagennr]' ";
       $res = mysql_query($sql) or die(mysql_error());
 
       while ($shown = mysql_fetch_array($res))
       {
           $positie = $shown['positienummer'];
           echo"pos: $positie";
 
       }
 
//       $sql = "INSERT INTO winkelwagenregels (winkelwagennummer, positienummer, merk, omschrijving, vendorpartnr, apn, leveranciernr, aantal, stuksprijs) VALUES
//                                             ('$wagennr', '$aantal', '$merk', '$omsch', '$vendor', '$levprod', '$levernr', '$val', '$pr')";
//       $resultaat = mysql_query($sql) or die (mysql_error());
 
   }
}
?>