Things to note about the is_file() and is_dir() functions when traversing directories

The result only shows that "footer.html" is a file, and the others have become directories: is a directory. It's a directory.. Is directory a footer.html Is the directory header.html Is the directory login_function.files.php Is the directory mysqli_connect.php It’s the directory style.css

The reason is that "$f" cannot be used directly in is_file and is_dir. This will be regarded by PHP as the file in the root directory, and there is the file footer.html in the root directory, so this file will be displayed correctly. Others don't. Just modify the code to the following content.