Incorrect highlighting of superglobals and predefined constants

ATM, php-mode treats all constants and superglobals as case-insensitive, i.e. `$_server` or `pHp_VeRsIon` are highlighted as if the were correct, but PHP will only throw errors if you run this.

From what I've gathered from the PHP documentation, only [the eight magic constants](http://php.net/manual/en/language.constants.predefined.php) are case-insensitive. All [superglobals](http://www.php.net/manual/en/language.variables.superglobals.php) and [predefined constants](http://www.php.net/manual/en/reserved.constants.php) and the ones from [core extensions](http://www.php.net/manual/en/extensions.membership.php#extensions.membership.core) are case-sensitive, and should only be highlighted if they are all caps