function getRemoteIPAddress() {
    $ip = $_SERVER['REMOTE_ADDR'];
    return $ip;
}

The above code will not work in case your client is behind proxy server. In that case use below function to get real IP address of client.
function getRealIPAddr()
{
    if (!empty($_SERVER['HTTP_CLIENT_IP']))   //check ip from share internet
    {
        $ip=$_SERVER['HTTP_CLIENT_IP'];
    }
    elseif (!empty($_SERVER['HTTP_X_FORWARDED_FOR']))   //to check ip is pass from proxy
    {
        $ip=$_SERVER['HTTP_X_FORWARDED_FOR'];
    }
    else
    {
        $ip=$_SERVER['REMOTE_ADDR'];
    }
    return $ip;
}