PHP __ (double underscore) means anything?

[AthlonInside]

I open up some open source PHP codes, even I have been coding for sometime, but I am confused to see them using __ (double underscore)

What does it do?

[R e b r a n d t]

Well, it depends where double underscore is being used?

Some coders use underscore to identify some types of variables: $__constant, $simplevariable.

PHP itself has some use double underscore and usualy it means some *extra* or *magic* functionality:
[php.net...]