any change to see php source?

Dear Sirs, I am studying on php. And the book say, when the server see php extension it is running php file and giving results to the visiting browser. But as I see related directories are normal directory, doesn' t like cgi-bin directories. Is there any change to see php file source such as file open function or something else? I am not going to somebody else file. Just I want to be sure how secure the php? Any experience? Best Regards