I have a php script that takes two PDF files and merges them into one file. I am using this in conjunction with the insert from URL script step. 

The way the script works is it takes two PDFs in container fields and base64encode each files. Then, those two base 64 strings are POSTed via insert from URL.

I've tested the script forwards and backwards and it does combine the PDF files reliably. The issue is when posting from FileMaker. What typically happens is the first page is combined twice and that's my resultant PDF. 

My php script checks that both inputs are different and throws an error is they are identical. I don't get any errors there.

Is there a way to POST a file from insert from URL?

I concatenate 2 PDF files using ghostscript on the Mac, not sure how to do that on Windows, should be a similar way.

If you like your PHP approach I do recommend looking into the way I write about in my blog: http://wethecomputerabusersamongst.blogspot.com/2013/10/execute-php-script-from-filemaker-with.html?q=mercutio