Choosing a proper PHP framework is difficult (see this). Fortunately, at last, I make a decision that I'm going to learn CakePHP at first. CakePHP is small and easy to use. And there are many free tutorials on the Internet. So I think it is not hard to get help. Also, I used FleaPHP which is developed by a Chinese programmer half a year ago. I feel CakePHP is similar to FleaPHP, and obviously CakePHP is much better and more powerful. It seems to be easier to learn than Symphony or Zend Framework for me.
I downloaded it just now and installed it on my computer according to its manual. It's very simple, just unpack the zip file into a sub-directory located in www root, for example, "cake". Then run your Apache, enter "http://localhost/cake/" in your browser's address bar. You will see a welcome page. If you don't see it, please refer to the manual book. I used XAMPP on Windows, a suit of Apache, PHP and MySQL. CakePHP runs well on it.
Now it is 22:50. I'm going to sleep. I will learn CakePHP formally next day and write some notes here.