function get_ip()
{
	static $realip = NULL;
	 
	if ($realip !== NULL)
	{
		return $realip;
	}
	 
	if (isset($_SERVER))
	{
		if (isset($_SERVER['HTTP_X_FORWARDED_FOR']))
		{
			$arr = explode(',', $_SERVER['HTTP_X_FORWARDED_FOR']);
	 
			/* 取X-Forwarded-For中第一个非unknown的有效IP字符串 */
			foreach ($arr AS $ip)
			{
				$ip = trim($ip);
	 
				if ($ip != 'unknown')
				{
					$realip = $ip;
	 
					break;
				}
			}
		}
		elseif (isset($_SERVER['HTTP_CLIENT_IP']))
		{
			$realip = $_SERVER['HTTP_CLIENT_IP'];
		}
		else
		{
			if (isset($_SERVER['REMOTE_ADDR']))
			{
				$realip = $_SERVER['REMOTE_ADDR'];
			}
			else
			{
				$realip = '0.0.0.0';
			}
		}
	}
	else
	{
		if (getenv('HTTP_X_FORWARDED_FOR'))
		{
			$realip = getenv('HTTP_X_FORWARDED_FOR');
		}
		elseif (getenv('HTTP_CLIENT_IP'))
		{
			$realip = getenv('HTTP_CLIENT_IP');
		}
		else
		{
			$realip = getenv('REMOTE_ADDR');
		}
	}
	 
	preg_match("/[\d\.]{7,15}/", $realip, $onlineip);
	$realip = !empty($onlineip[0]) ? $onlineip[0] : '0.0.0.0';
	 
	return $realip;
}

即使用户通过代理服务器上网，也能查出他的IP。但是要求代理服务器的类型是transparent类型。如果代理服务器是anonymous或者是high anonymous类型，HTTP_X_FORWARDED_FOR也无能为力了。