在哪里声明一个全局变量 Where do I declare a global variable?

正确答案 Personally, I would either write module that abstracts away the need for a global variable, or at least has a getter function for retrieving it. Something like

function foo_get_the_service ()
{
  static $service;

  if (! isset($service)) {
    $service = foo_init_the_service();
  }

  return $service;
}

Several Drupal API functions do something similar (eg, menu_get_active_trail() andmenu_set_active_trail());

And then preferably, your module would also have helper functions so you don't need to access the global directly. If something really does need the global, then you would use the getter.

If your global is an instance of a proper PHP class, then you could look into using a real singleton.