Fout in session.

 <?
 $klantnummerl = $_SESSION['loggedin']; //klantnummer opvragen
 ?>
<?php 
mysql_connect('localhost','root',''); 
mysql_select_db('') or die (mysql_error()); 

if(mysql_result(mysql_query("SELECT COUNT(klantnummer) FROM `adverterders`"),0) == 0) 
  { 
    echo 'Er staan nog geen berichten in de database'; 
  }
  
  ?> 
  <?php 
  $qSelect_berichten  = mysql_query('SELECT * FROM `adverterders` WHERE klantnummer = $klantnummerl') or die (mysql_error()); 
  while($aBerichten = mysql_fetch_array($qSelect_berichten))  
     
 $advid = $aBerichten['adv']; ?>

<?
 $klantnummerl = $_SESSION['loggedin']; //klantnummer opvragen
 ?>
<?php 
mysql_connect('localhost','root',''); 
mysql_select_db('') or die (mysql_error()); 
 
if(mysql_result(mysql_query("SELECT COUNT(klantnummer) FROM `adverterders`"),0) == 0) 
  { 
    echo 'Er staan nog geen berichten in de database'; 
  }
 
  ?> 
  <?php 
  $qSelect_berichten  = mysql_query('SELECT * FROM `adverterders` WHERE klantnummer = $klantnummerl') or die (mysql_error()); 
  while($aBerichten = mysql_fetch_array($qSelect_berichten))  
 
 $advid = $aBerichten['adv']; ?>

$qSelect_berichten  = mysql_query('SELECT * FROM `adverterders` WHERE klantnummer = \''.mysql_real_escape_string($klantnummerl).'\'') or die (mysql_error());

$qSelect_berichten  = mysql_query('SELECT * FROM `adverterders` WHERE klantnummer = \''.mysql_real_escape_string($klantnummerl).'\'') or die (mysql_error());