PHP is both a general programing language and a server-side scripting language. PHP origially stood for Personal Home Page, now it is known as PHP: hypertext Preprocessor. It was created by Rasmus Lerdorf in 1994. Lerdorf said “I don’t know how to stop it, there was never any intent to write a programming language. I have absolutely no idea how to write a programming language, I just kept adding the next logical step on the way.” PHP code can be embedded into HTML code.
(image via http://www.w3schools.com/php/default.asp)
PHP Tutorial Video
PHP can also be combined with web template systems or web frameworks. It is processed by an interpreter applied as a CGI (Common Gateway Interface). Then the web server uses the results by combining them between the executed and interpreted codes. Those codes can be anything from data to images. PHP can also be used with a CLI (command-line interface) as well. It can be developed on most servers for free.
The source of information used in this site was derived from wikipedia.org