atal error: Uncaught Error: Object of class PDOStatement could not be converted to string in code example

Example: object of class pdostatement could not be converted to string

In the comments, you show the following:

$query = $db->query('SELECT MAX( Bestelnummer ) FROM Bestellingsdetail');
$query->execute();
$max = $query;
$max++;
This is not how you get the result from a query. You are setting $max to a PDOStatement object. You need to fetch() the result in order to use it.

// I've added "AS maxval" to make it easier to get the row
$query = $db->query('SELECT MAX(Bestelnummer) AS maxval FROM Bestellingsdetail');
$max_row = $query->fetch(PDO::FETCH_ASSOC);

$max = $max_row['maxval'];
$max++;
Docs: http://www.php.net/pdo.query

P.S. $query->execute(); is only needed for prepared statements. query() will execute the query immediately.