how to set ignoreReadOnlyFields for addRequest in 2012 PHP Toolkit?

I've been able to initialize an invoice from a sales order using 2012 PHP Toolkit (see code below). The invoice object returned looks fine.

However, when I try to add the invoice, I'm getting
INSUFFICIENT_PERMISSION
You do not have permissions to set a value for element subtotal due to one of the following reasons: 1) The field is read-only; 2) An associated feature is disabled; 3) The field is available either when a record is created or updated, but not in both cases.

I've read the Platform Guide examples for setting ignoreReadOnlyFields in other languages, and in posts for PHP for pre-2012 endpoints. But, what I've tried for the 2012 PHP Toolkit 2012 has not yet worked. I can see the preference is not getting set in the header. The only header fields seem to be the ones set in NSconfig.php