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 are case-insensitive. All superglobals and predefined constants and the ones from core extensions are case-sensitive, and should only be highlighted if they are all caps