pass-thru post variables?

Hi: I've got html page "A," Which posts some variables to php page "B." I'd like PHP page "B" to process those variables, create some additional variables, and post the whole bunch to PayPal CGI Script "C". I'd like this PHP script to operate seemlessly, so that the user feels like they are going straight from Html page "A" to PayPal CGI script "C". Also, I don't want to put the variables up in the URL ... I want to use the post method. How can I post variables from a php script without requiring user input? Any ideas? Thanks in advance ...