Bewerken lukt niet

<?php

define('HOST', 'localhost');
define('USER', 'root');
define('PASS', '');
define('DBNAME', 'test');

class Singleton {

	private static $_instace;
	private $conn;

	private function __construct() {
		$this->conn = mysql_connect(HOST, USER, PASS);
		mysql_select_db(DBNAME);
	}

	public static function getconnect() {
		if (!self::$_instace) {
			self::$_instace = new Singleton();
		}
		return self::$_instace;
	}

	public function mysql_uitvoeren($sql) {
		$sql = ltrim($sql);
		$arr_ophalen = array();

		if (!empty($sql) && isset($sql)) {
			$string_sql = strtoupper(substr($sql, 0, 6));
			if ($string_sql == 'SELECT') {
				$statement = mysql_query($sql, $this->conn);
				while ($rij = mysql_fetch_assoc($statement)) {
					$arr_ophalen[] = $rij;
				}
				if (count($arr_ophalen) > 0) {
					return $arr_ophalen;
				} else {
					return false;
				}
			} elseif ($string_sql == 'CREATE') {
				if (mysql_query($sql, $this->conn)) {
					return true;
				} else {
					return false;
				}
			} elseif ($string_sql == 'UPDATE' || 
$string_sql == 'DELETE' || $string_sql == 'REPLACE') {
				if (mysql_query($sql, $this->conn)) {
					return mysql_affected_rows();
				} else {
					return false;
				}
			} else {
				if (mysql_query($sql, $this->conn)) {
					return mysql_insert_id();
				} else {
					return false;
				}
			}
		} else {
			return false;
		}
	}

	private function __close() {
		
	}

}

?>

<?php

require_once 'Singleton.php';

class Afdeling {

	public $id, $afdeling;
	protected $connect;
	protected $sql;

	public function __construct() {
		$this->connect = Singleton::getconnect();
	}

	public function __destruct() {
		$this->connect = null;
	}

	public function afdeling_toevoegen($afdeling){
            
            $afdeling="INSERT INTO afdeling SET afd_naam='".$afdeling."'";
            return mysql_query($afdeling);
            
        }
	
	public function afdeling_bijwerken($afdeling){
            
            $afdeling="UPDATE afdeling SET afd_naam='".$afdeling."' WHERE `afdeling`.`id` =".$id;
            return mysql_query($afdeling);
            
        }
	
	
	public function verwijderen_afdeling($id){
		
			$afdeling="DELETE FROM `test`.`afdeling` WHERE `afdeling`.`id` =".$id;
			return mysql_query($afdeling);
		
		}
		
		public function tel_afdeling() {
		$afdeling = 'SELECT COUNT(*) as totaal FROM afdeling';
		return mysql_query($afdeling);
		
	}
	
		public function select_afdeling() {
		$afdeling = 'SELECT afd.id, afd.afd_naam AS Afdeling
		FROM afdeling afd';
		return mysql_query($afdeling);
	}
	
	
	public function select_afdeling_id($id) {
		$afdeling = 'SELECT * FROM afdeling WHERE id= ' .$id;
		return mysql_query($afdeling);
	}
	
	
	
	
	public function select_afdelingen() {
		$afdeling = 'SELECT * FROM afdeling';
		return mysql_query($afdeling);
	}

}

?>

<?php
    require_once 'class/afdeling.php';
      
      $afdeling1 = new Afdeling();
	  
      $alle_afdelingen = $afdeling1->select_afdeling();
	  

      echo '<table border="0" cellspacing="4" cellpadding="4"><tr>';
	  $intRows = 0;

	  while($a=mysql_fetch_assoc($alle_afdelingen))
	  {
		 $bewerken = '<a href="afdeling_bewerken_handler.php?id=' . $a['id'] . '"><img src="Images/wijzigen.gif" height="23" width="23"></a>';
		 echo '<td>';  
		 $intRows++;
         echo '<td>' . $a['Afdeling'] . '</td>';
         echo '<td>' . $bewerken . '</td>';
         echo'</td>';  
		if(($intRows)%2==0)  
		{  
		echo'</tr>';  
		}  
      }

      echo '</table>';
      ?>

        <div id="admin-container2">
    
<?php
require_once 'class/afdeling.php';

if (isset($_POST['bijwerken'])) {
	$afdeling = new Afdeling();
	$afdeling->id = $_POST['id'];
	$afdeling->afdeling = $_POST['naam'];
	if ($afdeling->afdeling_bijwerken()) {
		header('Location:overzicht_afdelingen.php');
	} else {
		header('Location:overzicht_afdelingen.php');
	}
}
?>

<?php
if (!empty($_REQUEST['id'])) {
	$afdeling1 = new Afdeling();
	$id = $_REQUEST['id'];
	
	$afdeling = $afdeling1->select_afdeling_id($id);
	$afdeling = $afdeling[0];
} else {
	header('Location:overzicht_afdelingen.php');
}
?>

<form method="post" action="afdeling_bewerken_handler.php">

<table border="0">
	<tr>
            <td colspan="2"><h15>Afdeling bewerken</h15><br><br></td>
	</tr>
	<tr>
            <td>Naam Afdeling:</td>
		<td>
			<input type="text" name="naam" size="20" 
			maxlength="20" value="<?php echo $afdeling['afd_naam']; ?>"/>
		</td>
	</tr>
	<tr>
		<td></td>
		<td><input type="submit" name="bijwerken" value="Update"  /></td>
	</tr>
</table>
<input type="hidden" name="id" value="<?php echo $afdeling['id']; ?>"/>
</form>
<br><?php require_once 'menu/cat_menu.php'; ?>
</body>
</div>