Build a XML using a webservice

I already have a PHP system to create invoices: customer´s catalog, product's catalog, categories, PDF generation, etc. However, there is a need to add a digital signature to the invoice. That signature is the answer of a webservice. What you send yo the webservice is an XML with a given XSLT definition. So I need you to integrate that piece of code. Please write PM for further discussion, code samples, webservice call and a demo of the system that already creates invoices. Let me know your comments, Joel Sanchez