Last slash in URL matters?

[Earnan]

I have an interesting problem. I have been working on my own Session based security process, and I have a particular page that I really want locked down.

My problem is inconsistent application of the security code.

If I go to the url www.whatever.com/mypage.php then the code works fine.
If I got to www.whatever.com/mypage.php/ then the security code does not run.

Process:
mypage.php contains an Include for the security page, code to run the security check, and an include for the actual mypage that I want to display if the security check passes.

When I have the additional slash on the end, it appears to completely ignore the session based security requests.

Any ideas (both how to ensure that my site is secure and why this is)?
Thank you.

[NogDog]

Well, the trailing "/" should mean that the web host loads the default page in the directory called mypage.php, not the mypage.php file in the web root directory. Do you have some funky .htaccess stuff going on?

[Earnan]

No, my host does not allow .htaccess (at least the commands I have tried).