Fix "not supported" for variables

We support all global variables, so just return true for them

Checking if they are set doesn't work with variable variables inside function
http://3v4l.org/WcL7P

Proof that we support everything:
http://3v4l.org/sUIPi#vhhvm-330

Undefined for
 * $php_errormsg
 * $http_response_header

because of 3v4l.org (php doesn't find them either)

We support them in reality

Fixes #396