Get PDF invoice from Quickbooks Online

Hi! I'm a programmer myself but for some reason I just can't figure out how to download a PDF version of an invoice from the sandbox environment. First, yes, it's possible since revision 82 of the V3 API. But the function isn't yet integrated in the object model of the SDK, so we have to go through cURL to call the URI defined in the API Explorer. I think I got all the sequence right to make the call, but it doesn't seem to like my Oauth_signature parameter. I provide my code, just need to be revised and tweaked. All the access keys are there in the code. The code is done at first to get the standard XML response from the invoice entity... with 2 lines commented on how to modify the code to get the PDF version of the invoice. I included at the bottom of the PHP code, in a large comment, the output I get from that script. It should be very easy for someone with experience with the Quickbooks Online API to see what's wrong and fix my error(s). Once I have this issue fixed, I'll be able to complete the application I'm trying to do.