We have a web site where all the files are pure html, but we are using extension php because eventually these will contain php code.
The site is structured with folders for each site section; for example, the "About" section is in folder about, etc.
The main page of the about section is about/index.php. The link is: <a href="about/index.php">
All of the links to php files in subfolders produce "No input file specified. "