How to get the service's X509 certificate?

Many PHP developers who are trying to write a secure PHP client using WSF/PHP, find it a bit difficult to understand "How to obtain the certificate of the service". The service certificate is required to encrypt the content and/or verify the service's signature. The X509 certificate used by the service can be hosted somewhere by the service writer, so that it can be easily downloaded. But in some cases it is not. We are supposed to extract it. This will tell you how to do that.