Insert does not pass new record id next page

I'm new to WebAssist, so the answer may be obvious...

When I set up an Insert record form using MySQLi Server Behaviours, I can't get the new record ID passed to the next page. The help says to save this is a session variable, but when I add this as part of the next page URL I get a PHP error and the page won't even load. This is the code:

$InsertQuery->saveInSession("con_id");
$InsertQuery->execute();
$InsertGoTo = "contact_details.php?id_usr=\" . $_SESSION['con_id'] . \"";

If I remove the session parameter from the URL, the form works, the record is inserted, but the PHP session value is about 26 characters long and bears no relation to the record id, which is the value 25 (i.e. only 2 characters long).

What am I doing wrong?

All help welcome, thanks.

PS: I am using DW6 on Windows Vista.