.php page failing to load

Why would something like this happen. Without having made any changes to any files, today suddenly .php pages in a photo gallery in the site are not loading in the browser window but instead are being downloaded as a file. In Firefox for example I get this message:

You have chosen to open
which is a: application/x-http-php
from: http://mysite.org

What should Firefox do with this file?

I have other pages in the site, .php pages that are working just fine. Any ideas as to what is causing this and how to fix it?

Thank you.

Check your .htaccess file for incorrect directives.

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