PHP (PHP: Hypertext Preprocessor) is an open source, reflective programming language. Originally designed as a high level scripting language for producing dynamic Web pages, PHP is used mainly in server-side application software but can be executed from a command line interface or a stand alone GUI. Here is several websites where you can start to learn PHP :
The PHP Resource Index - php.resourceindex.com Site with complete script, code snippets, documentation and other PHP related resources.
PHP Builder - www.phpbuilder.com The source for php developers with a variety of source codes and tutorials.
PHP Freaks - www.phpfreaks.com A community designed to assist developers with PHP and MySQL related information. Includes tips, tutorials, forums, articles, code examples and manuals.
ONLamp.com PHP DevCenter - www.onlamp.com/php/ O'Reilly Network's source for PHP developer news and information.
The PHP Extension and Application Repository (PEAR) - pear.php.net A framework and distribution system for reusable PHP components.