PHP is a server-side scripting language for creating dynamic Web pages. You create pages with PHP and HTML. When a visitor opens the page, the server processes the PHP commands and then sends the results to the visitor's browser, just as with ASP or ColdFusion. Unlike ASP or ColdFusion, however, PHP is Open Source and cross-platform. PHP runs on Windows NT and many Unix versions, and it can be built as an Apache module and as a binary that can run as a CGI. When built as an Apache module, PHP is especially lightweight and speedy. Without any process creation overhead, it can return results quickly, but it doesn't require the tuning of mod_perl to keep your server's memory image small.
In addition to manipulating the content of your pages, PHP can also send HTTP headers. You can set cookies, manage authentication, and redirect users. It offers excellent connectivity to many databases (and ODBC), and integration with various external libraries that let you do everything from generating PDF documents to parsing XML.
PHP goes right into your Web pages, so there's no need for a special development environment or IDE. You start a block of PHP code with <?php and end it with ?>. (You can also configure PHP to use ASP-style <% %> tags or even <SCRIPT LANGUAGE="php"></SCRIPT>.) The PHP engine processes everything between those tags.
PHP's language syntax is similar to C's and Perl's. You don't have to declare variables before you use them, and it's easy to create arrays and hashes (associative arrays). PHP even has some rudimentary object-oriented features, providing a helpful way to organize and encapsulate your code.
Although PHP runs fastest embedded in Apache, there are instructions on the PHP Web site for seamless setup with Microsoft IIS and Netscape Enterprise Server. If you don't already have a copy of PHP, you can download it at the official Web site. You'll also find a manual that documents all of PHP's functions and features.