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8 ” HTTP R equests The HTTP protocol is intended to give two parties a common method of communication: web clients and web servers. Clients are programs or scripts that send requests to servers. Examples of clients include web bro wsers, such as Internet Explorer and M ozilla Firefox, and crawlers, like those used by Yahoo! and Google to expand their search engine offerings. Servers are programs that run indefinitely and do nothing but receive and send responses to client requests. P opular examples include Microsoft IIS and the Apache HTTP Server. You must be familiar enough with the anatomy and nuances of HTTP requests and responses to do two things. First, you must be able to configure and use your preferred client to view requests and responses that pass between it and the server hosting the target application as you access it. This is essential to developing your web scraping application without expending an excessive amount of time and energy on your part. Second, you must be able to use most of the features offered by a PHP HTTP client library. I deally, you would kno w HTTP and PHP well enough to build your o wn client library or fix issues with an existing one if necessary. In principle, ho wever, you should resort to finding and using an adequate existing library first and constructing one that is reusable as a last resort. W e will examine some of these libraries in the next few chapters. Supplemental References This book will co ver HTTP in sufficient depth as it relates to web scraping, but should not in any respect be considered a comprehensive guide on the subject. H ere are a few recommended references to supplement the material co vered in this book. • RFC 2616 HyperText Transfer Protocol – HTTP/1.1 (http://www.ietf.org/rfc/rfc2616.txt) • RFC 3986 U niform Resource I dentifiers (URI): Generic Syntax (http://www.ietf.org/rfc/rfc3986.txt) • “HTTP: The Definitive Guide” (ISBN 1565925092)
HTTP ” 9 • “HTTP P ocket Reference: HyperText Transfer Protocol” (ISBN 1565928628) • “HTTP Developer’s Handbook ” (ISBN 0672324547) • Ben Ramsey’s blog series on HTTP (http://benramsey.com/http-status-codes) GET Requests Let’s start with a very simple HTTP request, one to retrieve the main landing page of the Wikipedia web site in English. GET /wiki/Main_Page HTTP/1.1 Host: en.wikipedia.org The individual components of this request are as follo ws. • GET is the method or operation. Think of it as a verb in a sentence, an action that you want to perform on something. Other examples of methods include POST and HEAD . These will be co vered in more detail later in the chapter. • /wiki/Main_Page is the U niform Resource I dentifier or URI. It pro vides a unique point of reference for the resource, the object or target of the operation. • HTTP/1.1 specifies the HTTP protocol version in use by the client, which will be detailed further a little later in this chapter. • The method, URL, and HTTP version collectively make up the request line, which ends with a (carriage return-line feed) sequence, which corresponds to ASCII characters 13 and 10 or U nicode characters U+000D and U+000A respectively. (See RFC 2616 Section 2.2 for more information.) • A single header Host and its associated value en.wikipedia.org follo w the request line. M ore header-value pairs may follo w . • Based on the resource, the value of the Host header, and the protocol in use (HTTP, as opposed to HTTPS or HTTP o ver SSL),
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