Please help me. How start PHP. I have complete some HTML language and CSS.
How about buying a book on PHP/MySQL? There are some online tutorials which you could read too. Start by installing a stack. XAMPP or WAMP.
Learn PHP from code academy! It's pretty easy. Php is Hyper Text Preprosser lanuages which means you can't execute your php functions with html events(onclick, onhover, mousein, mouseout) , you probably will gonna need to learn ajax to execute php functions with POST SEND GET methods which you will gonna learn in PHP. MYSQL is pretty simple, i learnt it in like a week browsing disucssion on stackoverflow. it took me a day to figure out how to connect my databases to php code and how to execute php functions with ajax!
Do all of the above as suggested, but don't get overly confident after learning how to echo "hello world". PHP is the most deceiving programming language. The outer shell is pretty shallow, but once you passed that outer shell, you could be lost into the black hole.
Just saying... :)