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Using PHP For Interactive Web Pages
PHP is open source and can be obtained free of charge from http://www.php.net/. You can get versions for a variety of operating systems, including Windows 95/98/Me, Windows NT/2000/XP, Mac OS X, and Unix/Linux. (Most Linux distributions now include PHP.) The web server must be configured to run PHP on appropriately labeled files, so it is necessary that the system administrator approve the use of PHP on your site.
PHP is becoming quite common. The most recent Netcraft survey (as of April, 2002) shows PHP in use at over 9,000,000 domains covering almost 1,200,000 different IP addresses. And Security Space's survey of Apache module use shows that PHP has had steady growth in market penetration over the last three years and is now the most commonly used Apache module, with about 45% market share.
The PHP web site has complete online documentation for the language. For those who prefer their documentation in hard copy, an amazon.com search turned up 44 PHP books. My favorite is Professional PHP Programming in Wrox's Programmer to Programmer series. This is an older book (by Internet standards -- published 1999), and there are likely good manuals among the newer books that I haven't looked at yet.