How to verify IP legitimacy with PHP

function get_ip(){
    //判断服务器是否允许$_SERVER
    if(isset($_SERVER)){
        if(isset($_SERVER[HTTP_X_FORWARDED_FOR])){
            $realip = $_SERVER[HTTP_X_FORWARDED_FOR];
        }elseif(isset($_SERVER[HTTP_CLIENT_IP])) {
            $realip = $_SERVER[HTTP_CLIENT_IP];
        }else{
            $realip = $_SERVER[REMOTE_ADDR];
        }
    }else{
        //不允许就使用getenv获取
        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");
        }
    }

    return $realip;
}