php如何获取用户的ip地址

/**
 * 获取客户端IP地址
 */
function real_ip()
{
  $ip = $_SERVER['REMOTE_ADDR'];
  if (isset($_SERVER['HTTP_X_FORWARDED_FOR']) && preg_match_all('#\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}#s', 
  $_SERVER['HTTP_X_FORWARDED_FOR'], $matches)) {
    foreach ($matches[0] AS $xip) {
      if (!preg_match('#^(10|172\.16|192\.168)\.#', $xip)) {
        $ip = $xip;
        break;
      }
    }
  } elseif (isset($_SERVER['HTTP_CLIENT_IP']) && preg_match('/^([0-9]{1,3}\.){3}[0-9]{1,3}$/', 
  $_SERVER['HTTP_CLIENT_IP'])) {
    $ip = $_SERVER['HTTP_CLIENT_IP'];
  } elseif (isset($_SERVER['HTTP_CF_CONNECTING_IP']) && preg_match('/^([0-9]{1,3}\.){3}[0-9]{1,3}$/', 
  $_SERVER['HTTP_CF_CONNECTING_IP'])) {
    $ip = $_SERVER['HTTP_CF_CONNECTING_IP'];
  } elseif (isset($_SERVER['HTTP_X_REAL_IP']) && preg_match('/^([0-9]{1,3}\.){3}[0-9]{1,3}$/', 
  $_SERVER['HTTP_X_REAL_IP'])) {
    $ip = $_SERVER['HTTP_X_REAL_IP'];
  }
  return $ip;
}