You are looking at it from the interpreter's perspective. I am looking at it from a document semantics perspective and what this means for more complex application environments.

Let me give you an example.

Suppose I have a GUI application that prints some XML fragments to standard output. The most natural way to do this in PHP is to write an XML pre-processor which happens to make decisions based upon operations in the GUI. In other languages you would print to standard output explicitly and the GUI would be primary. In PHP the XML output would be primary and the GUI would be secondary.

In fact your entire GUI is sort of contained in XML fragments to start with.