You're slightly misunderstanding what they're saying I think:
PHP may in future define constants which, unfortunately for such code, have the same name. It works because PHP automatically converts a bare string (an unquoted string which does not correspond to any known symbol) into a string which contains the bare string. For instance, if there is no defined constant named bar, then PHP will substitute in the string 'bar' and use that.
Thats different to the technique I advised. You're thinking of
but I was talking of
PHP are talking of defining constants instead of using a string if it exists. I am not demonstrating using a constant but using a string which (as quoted in my previous post) is treated as a string because its already inside a string. Strings are variable so PHP isn't suddenly going to decide that there is a constant half way through a string is it!