Rob Richards has posted about some updates that have been made to the WSE-PHP project to include support for dealing with encrypted SOAP messages. He talks some about the implementation and how it can help secure your application.

Support for dealing with encrypted SOAP messages was a feature that had been missing from my WS-* libraries pretty much due to my lack of time to implement it. When people ask for help interoperating with a SOAP server that requires encryption I typically just point them over to the WSF project. [...] I decided to finally set aside some time and implement the encryption/decryption portion of the library. Note that it was developed against some WCF services implementing message level security, so your mileage might vary against other platforms. Bug reports would be fastest way of getting any of those issues dealt with.

The WSE-PHP project lives on the Google Code site and provide support for a portion of WS-Addressing and WS-Security that can be used with ext/soap in PHP. TO use the library, you'll need to have the xmlseclibs library already installed.