Php (PHP: Hypertext Preprocessor) is a popular scripting language created in 1995 by Rasmus Lerdorfused. Currently it's one of the most popular scripting languages used to create dynamic pages by millions of websites online. The syntax of php is very easy to learn, combined with the fact that php is free software, and the fact that it contains 100s of built in functions makes php the perfect language for dynamic website development.
Recent studies estimate that about 35% of dynamic web pages are run using php, followed by 21% that are run by competing scripting language developed by microsoft: ASP (active server pages). This fact makes php an ideal language to learn, as high skilled php developers are in highest demand ever.
PHP comes preloaded with 100s of different functions, everything from sessions handling, text processing, to cookies handling functions, to database librararies for just about any DBM system out there. Documentation of all the available functions can be found on php.net.
PHP is typically on a "LAMP" system, LAMP standing for Linux, Apache (popular free web server), Mysql (popular free database system) and of course "P" which stands for PHP. Nearly all linux distributions come preinstalled with all "LAMP" components, for windows developers "WAMP" packages are available, which contain Apache, Mysql and PHP binaries in an easy to install package.