please excuse me but it's hard to make out what you mean here.
It looks like you mean "why can't I download a php file and not see the php code, only the html part?"
The reason (if this is what you mean) is that php is a server side scripting language. It stays on the server and only delivers it's tasks to the user, not it's source code. Unless you are the owner and uploader of the source code, you won't see it. This makes it extremely secure from prying eyes and hackers, if it's used right.
If this wasn't the case then a lot of online store owners would be in deep trouble.