Urgent: How do I disallow a php/html file to act as a folder?

I recently noticed that for some reason, if I type the filename as a folder ("http://localhost/folder/") it runs the nearest file called that. I have several .php on a server, which can run without the extention. ("http://localhost/folder/file"), but if I typed ("http://localhost/folder/file/") it'll still run that file. And to save time, I have made a header and footer for each of the pages I make, so it does not consume more time re-adding them everytime I create a new document. Is there a way that I can disable this? So that a file (php, html, jsp, etc.) can't be launched as a folder, but can still be launched without the file extension? So: http://localhost/folder/file works, http://localhost/folder/file.php works but: http://localhost/folder/file/ doesn't unless there is a folder called "file". I don't know if this is a PHP problem, or an Apache problem. Thanks, Dan