file_get_contents failed to open stream: HTTP request failed

Question

I am trying to use wp-authentication from another server. So I have login.php file:

<?php
    $content=file_get_contents("http://xxx/custom_login.php");
    echo $content;
?>

and custom_login.php, on another site:

<?php 
    require('wp-blog-header.php'); 
?>

I think this is where it breaks. Although, when called on its own, custom_login.php works fine.

Can I fix this?