What Is HTTP?
HTTP is to Web pages what FTP is to file access. It is the core protocol behind the World Wide Web. Every click on a Web page, every image that is displayed, every style sheet that is requested uses HTTP. It's not unfair to say that HTTP is the backbone of the Web. Even though you don't often think about it, for most users, they "use" HTTP more than any other Internet protocol.
Like most of the Internet, HTTP is an ASCII-based protocol that is clearly specified and easy to leverage. Even though PHP largely shields you from the bits that make up HTTP, understanding HTTP is an essential part of your development skills. After you've mastered HTTP, you'll be better able to build Web applications because what is going on behind the scenes will actually make sense.