Variabele post-button.

<?php if(isset($_GET['nr'])) { 
 if($_SERVER['REQUEST_METHOD']=='POST') { 
 
     $nr = $_GET['nr'];
    $klas = $_GET['klas'];
    $cat= $_GET['cat'];
                  $query = mysql_query("SELECT * FROM materiaal WHERE rubriek='" . $cat . "'") or die(mysql_error());
                $obj = mysql_fetch_object($query);
            echo'<BR><div class="blok">';
                    echo'Nr. ';
                    echo $nr;
                    echo' van klas ';
                    echo $klas;
                    echo' heeft het volgende materiaal mee: ';
                    echo'<BR>';
            foreach($_POST['gekozen'] as $obj->Materiaal_ID) {
                    echo '<b>';
                    echo  $voorwerpen[$obj->Materiaal_ID];
                    echo '</b><br></div><br>';            }
        
    } 
    }
?>
<form method="post">
<center>
  <table width="376">
    <tr>
      <th width="76" bgcolor="#666666" scope="col">Voorwerp</th>
      <th width="44" bgcolor="#666666" scope="col">Aantal</th>
      <?php if(isset($_GET['nr'])) { ?> <th width="150" bgcolor="#666666" scope="col">Kiezen</th><?php } ?>
    </tr>
    <?php 
        $cat= $_GET['cat'];
  $query = mysql_query("SELECT * FROM materiaal WHERE rubriek='" . $cat . "'") or die(mysql_error());
    while ($obj = mysql_fetch_object($query)) {
                ?>
    <tr>
      <td><center>
       <?php echo $obj->Omschrijving; ?>
      </center></td>
      <td><center>
        <?php echo $obj->Aantal_beschikbaar; ?>
      </center></td>
     <?php if(isset($_GET['nr'])) { ?> <td><center>
        <input type="submit" value="<?php echo $obj->Materiaal_ID; ?>" name="gekozen[<?php echo $obj->Materiaal_ID; ?>]" />
      </center></td><?php } ?>
    </tr>
    <?php
  }
  ?>
  </table>
 </form>
 <?php }
 }
 
?>