A PHP file is a web page that may contain PHP (Hypertext Preprocessor) code. It may include PHP functions that can process online forms, get the date and time, or access information from a database, such as a MySQL database.
The PHP code within the web page is processed (parsed) by a PHP engine on the Web server, which dynamically generates HTML. The HTML, which contains the web page content, is then sent to the user's web browser. Therefore, the user never sees the actual PHP code contained in the web page, even when viewing the page source.
Common PHP Filenames:
index.php - Often the default file to be loaded when a client Web browser requests a directory from a PHP-enabled Web server. For example, if http://sampledomain.com/ is requested, http://sampledomain.com/index.php is loaded.