PHP obtains the IP address, and the apache2 server runs without error

this ip The advantage of getting a function is apahce2 No error is reported at runtime . 

function getRealIp()
{
    $ip=false;
    if(!empty($_SERVER["HTTP_CLIENT_IP"])){
        $ip = $_SERVER["HTTP_CLIENT_IP"];
    }
    if (!empty($_SERVER['HTTP_X_FORWARDED_FOR'])) {
        $ips = explode (", ", $_SERVER['HTTP_X_FORWARDED_FOR']);
        if ($ip) { array_unshift($ips, $ip); $ip = FALSE; }
        for ($i = 0; $i < count($ips); $i++) {
            if (!eregi ("^(10│172.16│192.168).", $ips[$i])) {
                $ip = $ips[$i];
                break;
            }
        }
    }
    return ($ip ? $ip : $_SERVER['REMOTE_ADDR']);
}
echo getRealIp();

The following code page can get ip Address , but apache2 Operation error reporting

PHP Notice:  Use of undefined constant HTTP_X_FORWARDED_FOR - assumed 'HTTP_X_FORWARDED_FOR'

function getip()
{
        if (isset($_SERVER)) 
        {
                if (isset($_SERVER[HTTP_X_FORWARDED_FOR]) && strcasecmp($_SERVER[HTTP_X_FORWARDED_FOR], "unknown"))// agent 
                {
                        $realip = $_SERVER[HTTP_X_FORWARDED_FOR];
                } 
                elseif(isset($_SERVER[HTTP_CLIENT_IP]) && strcasecmp($_SERVER[HTTP_CLIENT_IP], "unknown"))
                {
                        $realip = $_SERVER[HTTP_CLIENT_IP];
                } 
                elseif(isset($_SERVER[REMOTE_ADDR]) && strcasecmp($_SERVER[REMOTE_ADDR], "unknown"))
                {
                        $realip = $_SERVER[REMOTE_ADDR];
                } 
                else
                {
                        $realip = 'unknown';
                }
        } 
        else
        {
                if (getenv("HTTP_X_FORWARDED_FOR") && strcasecmp(getenv("HTTP_X_FORWARDED_FOR"), "unknown"))
                {
                        $realip = getenv("HTTP_X_FORWARDED_FOR");
                }
                elseif(getenv("HTTP_CLIENT_IP") && strcasecmp(getenv("HTTP_CLIENT_IP"), "unknown"))
                {
                        $realip = getenv("HTTP_CLIENT_IP");
                } 
                elseif(getenv("REMOTE_ADDR") && strcasecmp(getenv("REMOTE_ADDR"), "unknown"))
                {
                        $realip = getenv("REMOTE_ADDR");
                } 
                else
                {
                        $realip = 'unknown';
                }
        } 
        return $realip;
}