Christopher Kunz has relaunched his php-security.net domain with a brand new blog and a new article posted today - "X.509 PKI login with PHP and Apache".
Since grid computing (that's what I'm currently doing) also is very much about Single-sign on and delegation of rights, username/password authentication schemes don't quite do it for us. Thus, a PKI (public key infrastructure) based on X.509 is employed.
He explains the acronym soup by defining the process as advanced means of "showing the web who you are". He explains some of the basics about the certificates and who good candidates for this type of authentication are. Then the tech starts - how to get Apache configured to use them and how to use them in PHP to authenticate the external user (requiring OpenSSL compiled in to access the needed functions).