SOAP + XMLSec?

Can anyone recommend a PHP SOAP library that covers digital signatures
and verification, or just with very good documentation?

I'm very new to SOAP, but need to write a signature-making SOAP client
and signature-verifying server as part of an integration project. I'm
currently exploring PEAR::SOAP but am finding its documentation
unsatisfactory.

I'm looking for a library that either has inbuilt signature/
verification capabilities, or allows me to feed outgoing payloads
through XMLSec before sending them to the server. Can I do this with
relative ease using PEAR::SOAP?

As an added complication, this project is aimed at a PHP 4 server. I
don't think we can upgrade it for this project.

I'm very grateful for any advice.