Stop to fast caching/parsing

Hi List, my Problem is, on the second line of my script I validate a session. If it's false, it dies. But in the first line, I require a header(html,body,title etc.). Now, if a session fails, my die-Message ends in a "<script>"-Part, so nobody sees it but gets an error by JS. Is there any way to stop parsing PHP before flushing out required headers? Thanks, Jan