PHP获取客户端IP地址

/**
 * @return string
 */
function get_ip()
{
	$ip = NULL;
	if(getenv('HTTP_CLIENT_IP') && strcasecmp(getenv('HTTP_CLIENT_IP'), 'unknown'))
	{
		$ip = getenv('HTTP_CLIENT_IP');
	}
	else if(getenv('HTTP_X_FORWARDED_FOR') && strcasecmp(getenv('HTTP_X_FORWARDED_FOR'), 'unknown'))
	{
		$ip = getenv('HTTP_X_FORWARDED_FOR');
	}
	else if(getenv('REMOTE_ADDR') && strcasecmp(getenv('REMOTE_ADDR'), 'unknown'))
	{
		$ip = getenv('REMOTE_ADDR');
	}
	else if(isset($_SERVER['REMOTE_ADDR']) && $_SERVER['REMOTE_ADDR'] && strcasecmp($_SERVER['REMOTE_ADDR'], 'unknown'))
	{
		$ip = $_SERVER['REMOTE_ADDR'];
	}
	return preg_match('/[\d\.]{7,15}/', $ip, $matchs) ? $matchs[0] : 'unknown';
}