This library introduces dozens of functions, all prefixed with __ to the global PHP namespace.
These routines are extensions, helpers, shorthand, wrappers around existing core PHP functions.
A prime example is json_encode which has some legacy defaults.
We simply change those, with our "magic" function, adding two __.
__json_encode($d);
It's (sorta) true! PHP says so.
PHP reserves all function names starting with __ as magical. It is recommended that you do not use function names with __ in PHP unless you want some documented magic functionality.
So, it's reserved, but explicitly for magic. And this library is doing "magic" in a similar way, just for functions not OO. And documented (sorta).