Programming PHP, 3rd Edition

HTTP Basics

The Web runs on HTTP, or HyperText Transfer Protocol. This protocol governs how web browsers request files from web servers and how the servers send the files back. To understand the various techniques we’ll show you in this chapter, you need to have a basic understanding of HTTP. For a more thorough discussion of HTTP, see the HTTP Pocket Reference by Clinton Wong (O’Reilly).

When a web browser requests a web page, it sends an HTTP request message to a web server. The request message always includes some header information, and it sometimes also includes a body. The web server responds with a reply message, which always includes header information and usually contains a body. The first line of an HTTP request looks like this:

GET /index.html HTTP/1.1

This line specifies an HTTP command, called a method, followed by the address of a document and the version of the HTTP protocol being used. In this case, the request is using the GET method to ask for the index.html document using HTTP 1.1. After this initial line, the request can contain optional header information that gives the server additional data about the request. For example:

User-Agent: Mozilla/5.0 (Windows 2000; U) Opera 6.0  [en]
Accept: image/gif, image/jpeg, text/*, */*

The User-Agent header provides information about the web browser, while the Accept header specifies the MIME types that the browser accepts. After any headers, the request contains a blank line to indicate the end of the header section. The request ...