passthru environment variables

I'm trying to use passthru in a PHP program to have an external program display some data. The problem is that I was trying to have the external program's environment pick up the form field variables automatically passed into the PHP program as shell environment variables. In other words, if a user typed "Smith" into the last_name field in a form, the PHP program called by that form starts out with $last_name = "Smith", and I would like the external program called by passthru within the PHP program to have a shell environment variable last_name = "Smith". Does anyone know an easy way to do this?