Problems can occur if you mix CURLOPT_URL with a 'Host:' header in CURLOPT_HEADERS on redirects because cURL will combine the host you explicitly stated in the 'Host:' header with the host from the Location: header of the redirect response.
In short, don't do this:
<?php
$host = "www.example.com";
$url = "http://$host/";
$headers = array("Host: $host");
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
Do this instead:
$host = "www.example.com";
$url = "http://$host/";
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $url);
?>