PHP means PreProcessor Hypertext PHP is an open source, light weight, easy to learn dynamic web scripting language. This is not a write place to explain you can find details on w3schools. You will get manual at php.net
Hypertext PreProcessor. It's a programming language, you can use it in web development by learning it.
PreProcessor Hypertext PHP is used in web development to output dynamic HTML pages, process forms, uploads or anything that is done on the website's server.
php is mainly used for database and creating dynamicly generated code on the fly based on user responses.