获取客户端IP的代码

90%的人可能都是用$_SERVER['REMOTE_ADDR']来获取客户端的ip，但这其实并不一定准--当客户端用代理时，将出错。正确的方法如下：

function ip_address() {
  static $ip_address = NULL;

  if (!isset($ip_address)) {
    $ip_address = $_SERVER['REMOTE_ADDR'];
    if (variable_get('reverse_proxy', 0) && array_key_exists('HTTP_X_FORWARDED_FOR', $_SERVER)) {
      // If an array of known reverse proxy IPs is provided, then trust
      // the XFF header if request really comes from one of them.
      $reverse_proxy_addresses = variable_get('reverse_proxy_addresses', array());
      if (!empty($reverse_proxy_addresses) && in_array($ip_address, $reverse_proxy_addresses, TRUE)) {
        // If there are several arguments, we need to check the most
        // recently added one, i.e. the last one.
        $ip_address = array_pop(explode(',', $_SERVER['HTTP_X_FORWARDED_FOR']));
      }
    }
  }

  return $ip_address;
}