Yes, it's true.. you should know HTML first - it will at least lend you in the learning of opening and closing tags.
It's not required, but it can be helpful. PHP itself goes far beyond HTML, and really if you want to know PHP you should learn some CGI/C++ code. At that rate, before you learn C++ you should know the basics of Pascal or Basic.
You can go back pretty far with what language to learn first, but if you have the desire to learn, you can definitely dive into PHP without knowing the other things. Generally people say you need HTML as there are many times where your PHP code will have HTML code inserted to it (in cases of writing tables, variables in alignment etc etc). So it can definitely be helpful.