I wanna learn how code php. Where should I start?

http://www.php.net

Official docs are always the best place to start

WebWishMaster is right, you won't find a better PHP resource than php.net but if you're not a programmer then the documentation found there can be little intimidating. If you really want to start at the beginning try something simple. I would suggest checking out Larry Ulman's "PHP For the World Wide Web" it's a Visual Quick Start by Peachpit Press. It won't tell you everything you need to know to be a great programmer but its a good place to start.

If you really want a great PHP/MySQL resource (other than their respective online documentations) check out "PHP and MySQL Web Development" by Luke Welling and Laura Thomson, this book never leaves my side when I'm coding.

Hope this helps.

You can even start off with downloading some ebooks off the internet or maybe you can pick up a book from your local book store (if you dont have knowledge on this subject pick-up some beginners book).
Hope this helps.