Core PHP is very basic PHP. It is used to create dynamic web pages. It works without any extra library. So it is very important to learn core or principle PHP programming to create dynamic web applications. You can see more about PHP from the official site of PHP Hypertext Preprocessor.
PHP can be used on all main operating systems, including Linux, many Unix variants (including HP-UX, Solaris and OpenBSD), Microsoft Windows, Mac OS X, RISC OS, and probably others. PHP has also hold up for most of the web servers now. This includes Apache, IIS, and lots of others. And this includes any web server that can utilize the Fast CGI PHP binary, like lighttpd and nginx. PHP works as either a module, or as a CGI processor .