i once used post variables to pass data between an asp page and a php page, and it worked exactly as one would expect it to: ie my asp page had a form i think and it's action was a php page, method was POST. I also think i used one once and did a meta refresh from an asp page to page.php?variable=value etc and then had access as you would expect (through $_POST["variable"] if globals are off, or just $variable if they're on).
That might be of use to you. I presume even if you're using IIS say to parse both the PHP and the ASP then you can use sessions stuff too, but only if it's on the same server, and even then it may need some tweaking. I'd be interested to hear if you manage it with just normal variables though.