This is where most people insert "RTFM", and that's a good place to start.
http://www.php.net
PHP is a pre-processor engine that allows your scripts to perform serverside tasks that are generated into common HTML and higher forms of web content.
If you read around these forums, you can learn a lot. Or do a google on PHP and you will find thousands of pages dedicated to subject.
For useful forums you could try:
http://www.vbulletin.com - you have to license this software
http://www.phpbb.com - which has improved drastically in the last year and is the best free alternative to vbulletin.
PHP unlike HTML, and like other programming languages isn't something you can learn overnight. There are enough snippets and samples out there that you can fake your way through, but it would do you best if you started at the source and progressed from there.
http://www.php.net