what is php file

PHP can generate dynamic page content.
PHP can create, open, read, write, delete, and close files on the server.
PHP can collect form data.
PHP can send and receive cookies.
PHP can add, delete, modify data in your database.
PHP can be used to control user-access.
PHP can encrypt data and apply validation.
PHP used to send and receive E-Mails.
PHP can be used to integrate with other third party tools.
PHP can output images, PDF files, and even Flash movies. You can also output any text, such as XHTML and XML.
  

PHP is a general-purpose scripting language especially suited to web development. It was originally created by Danish-Canadian programmer Rasmus Lerdorf in 1994; the PHP reference implementation is now produced by The PHP Group
Implementation language: C (primarily; some components C++)
Stable release: 7.4.11 / October 1, 2020; 5 days ago
Preview release: 8.0.0 Release Candidate 1 / October 1, 2020; 5 days ago
Typing discipline: Dynamic, weak since version 7.0: Gradual
Designed by: Rasmus Lerdorf
Developer: The PHP Development Team, Zend Technologies 

PHP is an acronym for "PHP: Hypertext Preprocessor" PHP is a widely-used, open source scripting language. PHP scripts are executed on the server.

PHP files can contain text, HTML, CSS, JavaScript, and PHP code.
PHP code is executed on the server, and the result is returned to the browser as plain HTML.
PHP files have extension ".php

PHP stood for Personal Home Page now it stands for Hypertext Pre-processor.
PHP is an open-source, interpreted, and object-oriented scripting language that can be executed at the server-side. 
PHP is well suited for web development. Therefore, it is used to develop web applications (an application that executes on the server and generates the dynamic page).
PHP was created by Rasmus Lerdorf in 1994 but appeared in the market in 1995.Most of PHP syntax is based on C. It is used to manage dynamic content, databases, session tracking, even build entire e-commerce sites.
PHP is powerful enough to be at the core of the biggest blogging system on the web (WordPress)!
PHP is deep enough to run the largest social network (Facebook)!
PHP is also easy enough to be a beginner's first server side language!