PHP session keeps setting the data to NULL

Hello, I have a class which handles session data to my database but every time I run PHP  and have session_start(), the session data in my database is set to 0.

I have no idea why its doing it.

My script is like this (sorry its a quite large class):
 

class FileSessionHandler
{
    private $_sess_db;

    function open($_sess_db, $sessionName)
    {
		$dsn = 'mysql:dbname=testDB;host=127.0.0.1';
		$user = '[hidden]';
		$password = '[hidden]';
	try {
		$_sess_db = new PDO($dsn, $user, $password);
		
	} catch (PDOException $e) {
		echo 'Connection failed: ' . $e->getMessage();
		exit;
	}

	$_sess_db->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);				
	$this->connection = $_sess_db;
        return true;
    }
    function close()
    {	
	$this->connection = null;
        return true;
    }
    function write($id, $data)
    {
	$access = time();
	$stmt = $this->connection->prepare("REPLACE INTO sessions (id,access,data) VALUES (?,?,?)");
		try {
		    $stmt->execute(array($id,$access,$data));
		} catch (PDOException $e) {
		    echo $e -> getMessage(); exit;
		}			
		if(!$stmt->rowCount()){
			return false; 
		}
			return true;
    }	
    function read($id)
    {	
		$stmt = $this->connection->prepare("SELECT data FROM sessions WHERE id = ?");
			try {
				$stmt->execute(array($id));
			} catch (PDOException $e) {
				echo $e -> getMessage(); exit;
			}		
		if(!$stmt->rowCount()){ return false; }
			$row = $stmt->fetch();
			return $row['data'];	
    }
    function destroy($id)
    {
		$stmt = $this->connection->prepare("DELETE FROM sessions WHERE id = ?");
			try {
				$stmt->execute(array($id));
			} catch (PDOException $e) {
				echo $e -> getMessage(); exit;
			}			
		if(!$stmt->rowCount()){ return ''; }
		return 1;	
    }
    function clean($max)
    {
		$old = time() - $max;
		$stmt = $this->connection->prepare("DELETE FROM sessions WHERE access < ?");
			try {
				$stmt->execute(array($old));
			} catch (PDOException $e) {
				echo $e -> getMessage(); exit;
			}					
		if(!$stmt->rowCount()){ return ''; }
		return 1;		
    }
}

$handler = new FileSessionHandler();
session_set_save_handler(
    array($handler, 'open'),
    array($handler, 'close'),
    array($handler, 'read'),
    array($handler, 'write'),
    array($handler, 'destroy'),
    array($handler, 'clean')
    );

session_start();

Now in my database the row has :
 

id : 2f534c82c004172c4fd49cfdd8d5f91a

access  : 1387054887

data : 1

 

So the session is holding the value 1. Now when i run the PHP script and check the row, the field data says 0, i don't know why it keeps updating it to 0 rather than keeping the value?

Please help been stuck on this all day!!