PHP如何获取用户的的真实IP地址

/*
 * 获取用户IP的函数
 */
function getIp(){
	$onlineip = "";
	if(getenv('HTTP_CLIENT_IP') && strcasecmp(getenv('HTTP_CLIENT_IP'), 'unknown')) {
	    $onlineip = getenv('HTTP_CLIENT_IP');
	} elseif(getenv('HTTP_X_FORWARDED_FOR') && strcasecmp(getenv('HTTP_X_FORWARDED_FOR'), 'unknown')) {
	    $onlineip = getenv('HTTP_X_FORWARDED_FOR');
	} elseif(getenv('REMOTE_ADDR') && strcasecmp(getenv('REMOTE_ADDR'), 'unknown')) {
	    $onlineip = getenv('REMOTE_ADDR');
	} elseif(isset($_SERVER['REMOTE_ADDR']) && $_SERVER['REMOTE_ADDR'] && strcasecmp($_SERVER['REMOTE_ADDR'], 'unknown')) {
	    $onlineip = $_SERVER['REMOTE_ADDR'];
	}
	return $onlineip;
}