MTOM: way of sending binary in soap

SOAP Message Transmission Optimization Mechanism (MTOM)
XOP (XML-binary Optimization Packaging)

RFC 2045 section 6.8 gives description of Base64 Content-Transfer-Encoding

Understanding MTOM

Advantages of MTOM
Introduction to MTOM: A Hands-on Approach (more advantages to MTOM)
Sending Files in Chunks with MTOM Web Services and .NET 2.0

Possible PHP library for MTOM

I have a client/soap server in AXIS2 that sends MTOM back and forth. This could serve as the web service. PHP needs to talk to the AXIS2 webservice.
A possible choice is WSO2 Web Services Framework/PHP Proven Interoperability

WSO2 WSF/PHP features proven interoperability with Microsoft .NET, WSO2 WSAS (Apache Axis2/Java based Web services application server) and other J2EE implementations. The basic SOAP level interoperability as well as WS-* specification implementations have been tested and proven to interoperate.

Attachments with Web Services and Clients
You can send and receive attachments with SOAP messages both in optimized as well as non optimized formats with MTOM support. Attachments with MTOM/XOP
-- problem seems to require that you know the mime type

Downloading a Binary File from a Web Service using Axis2 and SOAP with Attachments

This example uses SWA.