Back in the early days of the internet, more and more websites came online, many developers became fed up with having to endlessly write documents with similar information repeatedly in order to build a website.
When surfing the net have you ever noticed that there are often similarities in the header and footer of many websites that you visit? It used to be that website developers would have to copy and past information over and over again for all the pages on the site.
This poses a problem, if there was a change that needed to be made on all the pages of the site, all those documents would require opening and editing.
PHP basically is a hypertext preprocessed and its heavily used scripting language that was designed to make it possible for dynamic webpages to function. PHP code can be embedded inside the HTML source of the document.
The webserver and its processor will generate a web page based upon its interpretation of the given document. Knowing and understanding how to work with PHP can be a useful tool to have.
The process of compiling a PHP code is not the same as compiling a C++ program. For instance, the code is executed by the web server client when someone accesses the php coded page. In order for this process to be possible, you would need a webserver such as Apache.
It comes with most linux distros. You could get it from www.apache.org. It serves your php scripts for clients that are connecting to it.