PHP
From OLSEncyclopedia
PHP is a recursive initialism for PHP: Hypertext Preprocessor. It is a scripting language that allows a web server to perform relevant tasks before sending you a page, such as getting necessary information out of a database. PHP is one of the most widely used server-side scripting languages today, and is Open Source, meaning that if you were so inclined, you could download the source code and compile it yourself. The upside of this for most general purposes is that PHP is free (as in beer) to obtain and install on a web server.
PHP files can contain HTML. When the server is executing the PHP file, any text outside of PHP's own special code blocks is simply sent to the web browser as-is.
To actually insert some code into a PHP file, one would enclose it in a PHP block like such:
<?php
// Here's some PHP code
print("Hello world!");
?>
There are very many functions available in PHP, from the usual stuff like math functions, output to the browser, and reading/writing other files on the server, to image manipulation and database manipulation.
