How do these arguments work? [, int $status_code= 500 ] ?

Narf is correct. In docs, [] as a parameter denote an optional parameter (also why the comma is inside the [] because if you use the optional parameter, you need the comma to separate parameters).

This is how the official PHP docs (and most others) are written. Example: http://php.net/manual/en/function.html-e...decode.php