php code visibility

Thanks in advance for your time: I need some documentation and concrete knowledge that supports or disproves the theory that php code can be viewed directly if it is in the web (www / htdocs) directory instead of a secure sub-folder. Can a person, in fact, get the php document returned directly so that they can view the source code before it is parsed by the php parser? Please, if you have knowledge of where I may find the truth in this discussion, respond. RW