Php Curl doesn't work on just one website - php

Question

i've an error very very strange.
I need to access this website: http://sub.example.com/ with cURL. From localhost i've no problem as well as if i use cURL from command line or browser to access the website.
I'm using this code on my Siteground server:
<?php
$url="http://sub.example.com/";
$agent= 'Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; .NET CLR 1.0.3705; .NET CLR 1.1.4322)';
$ch = curl_init();
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
curl_setopt($ch, CURLOPT_VERBOSE, true);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_USERAGENT, $agent);
curl_setopt($ch, CURLOPT_URL,$url);
$result=curl_exec($ch);
var_dump($result);
?>
and i get $result = false.
If i use Try / Catch i get: Curl failed with error #6: Could not resolve host
I've also tried to substitute $url with another website's url and it always works (http and https, domain and subdomain, on my server and external)!
I spent already a day trying to solve this, please help me!
Thank you in advance, Luca

Answer 1

according to googledns, sub.example.com does not exist.
$ nslookup sub.example.com 8.8.8.8
*** google-public-dns-a.google.com can't find sub.example.com: Non-existent domain
Server: google-public-dns-a.google.com
Address: 8.8.8.8
the fact that it works for you on localhost means you have a weird DNS, or perhaps an entry for it in your hosts file , while your Siteground server does not.
anyway, Curl failed with error #6: Could not resolve host means the server's dns could not find sub.example.com, but luckily, if you know what ip you want to connect to, you can bypass the DNS entirely, by instead writing:
curl_setopt($ch, CURLOPT_URL,'http://93.184.216.34/');
curl_setopt($ch,CURLOPT_HTTPHEADER,array('Host: sub.example.com'));
(there is also a more correct-ish curlopt to tell curl which ip goes with wich domain, but the the name escapes me at the moment, and iirc, php has not added official support for it, so you'd have to use the magic value for it, rather than the defined name, but manually setting the Host: header is pretty easy anyway)