PHP is a server-side, cross-platform, HTML embedded scripting language. If you are completely new to PHP and want to get some idea of how it works, have a look at the Introductory Tutorial from the official PHP web site. Once you get beyond that, have a look at the example archive sites and some of the other resources available in the Links section of that site.
When it comes to Web development these days, you have a lot of options. Many of these methods involve preprocessing?that is, embedding code into HTML pages with special tags that signal to a preprocessor that they contain code, and that it should do something with it. Much like a CGI, this code is then run on the server, and it returns some content, which then assumes part of the shape of the resulting HTML page sent back to the browser. Both the open source scripting language PHP and languages within Microsoft's ASP.NET framework fall into this category; JavaServer Pages (JSP) and Perl/Mason operate this way as well.
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