How thinkphp gets the client IP, thinkphp gets ip_PHP tutorial

function get_client_ip($type = 0) {
  $type    = $type ? 1 : 0;
  static $ip =  NULL;
  if ($ip !== NULL) return $ip[$type];
  if (isset($_SERVER['HTTP_X_FORWARDED_FOR'])) {
    $arr  =  explode(',', $_SERVER['HTTP_X_FORWARDED_FOR']);
    $pos  =  array_search('unknown',$arr);
    if(false !== $pos) unset($arr[$pos]);
    $ip   =  trim($arr[0]);
  }elseif (isset($_SERVER['HTTP_CLIENT_IP'])) {
    $ip   =  $_SERVER['HTTP_CLIENT_IP'];
  }elseif (isset($_SERVER['REMOTE_ADDR'])) {
    $ip   =  $_SERVER['REMOTE_ADDR'];
  }
  // IP地址合法验证
  $long = ip2long($ip);
  $ip  = $long ? array($ip, $long) : array('0.0.0.0', 0);
  return $ip[$type];