PHP Reading a S/Mime email p7m attachment (Base64?)

Question

We have a system using Symantec's PGP Universal Web Messenger.

This means that emails are sent to me using s/mime encryption from a pkcs12 cert.

I'm having a bit of trouble reading the messages in PHP.

I have all the imap code but now how do I read the encrypted p7m attached file. Is it just Base64 encoded and signed?

EDIT: I have the public key.

Here is the mime info:-

Content-Type: application/pkcs7-mime; smime-type=enveloped-data;\r\n\tname="Message.p7m"\r\nContent-Transfer-Encoding: BASE64\r\nContent-Disposition: attachment; filename="Message.p7m"