Hypertext Pre-processor (PHPs) is a server-side scripting language, and server-sidescripts are special commands you must place in Web pages. Those commands are processed before the pages are sent from your Server to the Web browser of your visitor. A typical PHP files will content commads to be executed in the server in addition to the usual mixture of text and HTML (Hypertext Markup Language) tags.
When you type a URL in the Address box or click a link on a Web page, you’re asking a Web server on a computer somewhere to send a file to the Web browser (sometimes called a “client”) on your computer. If that file is a normal HTML file, it looks exactly the same when your Web browser receives it as it did before the Web server sent it. After receiving the file, your Web browser displays its contents as a combination of text, images, and sounds. In the case of an PHP page, the process is similar, except there’s an extra processing step that takes place just before the Web server sends the file. Before the Web server sends the HTML file to the Web browser, it runs all server-side scripts contained in the page. Some of these scripts display the current date, time, and other information. Others process information the user has just typed into a form, such as a page in the Web site’s guestbook
To distinguish them from normal HTML pages, PHP files are usually given the “.php” extension.