PHP file
From Wiki @ Karl Jones dot com
A PHP file is a text file which has the .php filename extension.
PHP code
PHP files contain PHP code, or a mix of PHP code and HTML.
PHP code
The PHP code goes inside one or more PHP code islands.
Web browsers and PHP
In order for a web browser to view a PHP file as a web page, the browser must make a request (computing) to a web server, so the server can process the PHP code and send a response to the server.
Web browsers cannot use the "Open File" command to view a PHP file. "Open File" works with HTML pages. But the PHP code in a PHP file must be processed by a web server. The browser does not process the PHP.