Some times you see when you open a folder, its showing list of pages contained in that folder instead of displaying index.php (or any similar default page). By default Apache shows index.html as the default page but if you want to include more sach pages, you need to make change in httpd.conf file.
Go to your Apache installation directory, and open conf folder. You will find httpd.conf file there.
Open this file to edit, and look for ‘DirectoryIndex’.
By default it will be
Change it to add your custom default page
DirectoryIndex index.html index.php index.html default.php default.htm
Save the file and restart the apache web server.
Now you should be able to see index.php by default.