PHP is a programming language designed to work with HTML, but unlike HTML, PHP has data processing capabilities. If you are familiar with HTML, you know that it is not really a programming language, but more of a rendering languageâ€”that is, HTML enables you to write Web pages using code that creates a pleasing (hopefully) display of text, graphics, and links within a browser. Although there are a few helpful features of HTML (such as the capability to cause a form submission), for the most part HTML does nothing programmatically. For example, there are no HTML commands that enable you to add two numbers together, or access a database.
If you remember the Web back in the early '90s, you may recall that early Web pages were made from HTML code written as plain text source files. When you made a connection to a Web site with your browser, the Web server software sent these plain text HTML files to be processed and rendered into Web pages. Your browser actually did the rendering process (and still does, to be sure), but if you clicked Viewâ‡¨Page Source, you'd see the raw HTML code.
So where does PHP fit in with HTML? PHP began as PHP/FI, developed in 1995 by Rasmus Lerdorf from some Perl scripts he had created for tracking accesses to his online resume. Eventually, Rasmus wrote an implementation in C, released the source code to the public, and by the beginning of 1998 version 3.0 of PHP was released (written by Rasmus Lerdorf, Andi Gutmans, and Zeev Suraski), the first version that is very similar to the current releases of PHP.
The main goal of PHP is to enable users to easily develop dynamic Web pages. The difference between dynamic Web pages and static Web pages is that the content and structure of dynamic Web pages may change each time they are accessed (that's what the back-end programming is for) whereas the content and structure of static Web pages is fixed and does not change unless the designer manually changes them.
Unlike many other languages, PHP can be embedded directly into HTML, making it quite easy for those familiar with HTML to grasp how to add back-end, programmatic functionality to their Web pages. This single capability is one of the most important factors in PHP's usefulness, and thereby its popularity. But have no doubt that PHP is growing into a much more full-features language going well beyond the initial intentions of its authors. PHP intends to be the primary language for a great variety of online and offline applications, and PHP5 is showing every sign of doing just that.
And you shouldn't forget how well PHP works with HTTP (HyperText Transfer Protocol), the communications protocol (pre-agreed format for data communications) for Web. Whenever you click a link or enter a Web address into your browser, a request in HTTP format is sent to the Web server, which responds by sending back the Web page. If the Web page isn't found, you'll probably get the "404 Not Found" error. Sending back the correct page or sending an error if the page is not found are HTTP functions. We discuss HTTP thoroughly in Chapter 2 because several important aspects of PHP applications depend on HTTP.
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