Urgent::Implementing PKI in PHP

We have a website in PHP where we want to implement PKI. The scenario is that there would be some registered dealers who woud have to buy Digital signatures. When they come to our website they would input several information through PHP designed forms and use the digital signatures to sign the data. We wanted to use OpenSSL for the same. However what I fail to understand is how to implement the OpenSSL functions for implementing the client side of this, which would involve encryption using Private key etc, since PHP would be parsed at the server only. Please respond, it is an urgent requirement Thx Renuka