WHAT IS PHP ?

“PHP, originally derived as Personal Home Page later on it was known as hypertext preprocessor”

PHP is a script language and interpreter that is freely available and used primarily on Linux Web servers... 

PHP is a server-side scripting language designed for web development but also used as a general-purpose programming language. PHP generally is used to program the back end of websites. It is completely server-side, the server reads PHP which generally tells it what HTML or JavaScript to display to the browser. When you submit a form it may be handled by PHP as well. So for any sort of interactive website or a site that needs a database or administration section PHP programming is essential.

“PHP code can be simply mixed with HTML code, or it can be used in combination with various templating engines and web frameworks.”