PHP is the most popular choice of language to create websites and web application. There are many free & paid Frameworks & CMS readily available to customize and can deploy websites in couple of hours. It was previously known as Personal Home Page, now stands for Hypertext Preprocessor. The Development was “accidentally” started in 1994 by by Rasmus Lerdorf as when he wrote some Common Gateway Interface(CGI) Programs in C. When Rasmus extended it to work with web-forms and communicate with databases, It was called PHP/FI (Personal Home Page/Forms Interpreter). Mainly, it is used to Create Dynamic, feature rich, complex websites. It is distributed with open source license and is free to download & setup. There is official documentation available online with references to everything used like functions, operators, database connectivity and much more.
PHP is a server side scripting language, which is the main target field. It works with a web server which serves pages to see in web browser. When a web browser sends a request to web server , the server sends HTML file and browser reads the file & renders the requested content. The HTML file with code gets parsed before sending to requester, so only output is shown.