how to: variable = php parsed file include

I've got a site that I've used php to include the navigation bar as a separate file. Within that nav-bar is a small php application. I have no problem including the php nav-bar file and it gets parsed by php and the application works. The problem is that I also want to use the same file include for the navigation in a dynamic "thank you" page that is generated by a php-based form processor. I'm using phorm.com's php form processor which I really like. The "dynamic" "thank you" page that it generates is actually a hard coded html page which phorm.php parses to replace form variables. I think my best way to do what I want is to have the script grab the nav-bar file, parse it through php, then take the string results and set a variable equal to that string. That way I can still use the script's built in parsing that replaces form variables in the hard coded "thank you" page. Does anyone know the syntax to do this? I understand the concept, but I'm way over my head. Thanks!