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.
