index.php usually contains initialization of all components of the site, loadable via structures include and require.
To begin with, it is the first file, which you can see by typing in the address bar of the domain name of the site. For example http://lphp.ru - is not specified the path to the index.php file, it makes the default server, as it initializes the index file. In this case, with the extension .php. There is no difference, you write the address of the site to index.php (http://lphp.ru/index.php) or without it, by default it is still opens the index.php!
How to download free index.php?
More precisely, you can say that index.php - a file that contains the PHP language code, only with this extension is possible, again, again by default, to use PHP scripts. There are options index configuration file, more correctly determine which file is open by default when you open the site directory. These settings can be made in the configuration apache file (httpd.conf), but as a rule, we generally use the services of hosts, so the server settings are not available to us, but this is not true. There is a file called .htaccess (with a point in front), designed to fine-tune your site. With it, and you can set the index file that you will open by default.