PHP has nothing to do with JavaScript. JavaScript is a client side scripting language, it is executed on the client (ie, the web browser of the user). PHP is a server side scripting language, which generates HTML (and possibly JavaScript) dynamically depending on the user's request. The client never sees any PHP code.
I'm not sure where you should start, but
www.php.net is probably a good site to bookmark. It has documentation on everything in PHP, all functions, etc.
Google is your friend. Search for PHP/JavaScript and tutorials. You can also purchase a book on the subject(s).