4042302 in PHP

4042302.php

<?php

  // Ideally read those 4 settings from environment variables
  // with `getenv` to make the script more portable. Only one
  // of the `$error_...` needs to have a value.

  // The base_url to which the path will be appended
  // when checking for a fallback. No trailing slash.
  $base_url = "https://example.com";
  // A URL to redirect to if the fallback didn't serve the page
  $error_redirect = '';
  // or a document to `include`
  $error_document = '';
  // or just a text message to `echo`
  $error_message = 'Not found';

  // If a base_url is set check if it hosts the page
  // and redirect if it does
  if (!empty($base_url)) {
    $url = "$base_url$_SERVER[REQUEST_URI]";
    $ch = curl_init($url);
    curl_setopt($ch, CURLOPT_NOBODY, true);
    curl_setopt($ch, CURLOPT_FOLLOWLOCATION, true);
    $response = curl_exec($ch);
    $response_code = curl_getinfo($ch,CURLINFO_RESPONSE_CODE);
    curl_close($ch);

    if ($response_code >= 200 && $response_code < 300) {
      header("Location: $url", true, 302);
      die();
    }
  }

  // If we reached here, it means we either didn't have
  // a base_url set or the base_url didn't host the page
  // so we 404 ourselves. We'll use a similar pattern as
  // Apache ErrorDocument and either redirect, include
  // a specific script or display a message
  if (!empty($error_redirect)) {
    header("Location: $error_redirect", true, 302);
    die();
  } else if (!empty($error_document)) {
    include($error_document);
  } else {
    echo $error_message;
  }

README.md

4042302 in PHP

Implementation of the 4042302 concept in PHP, to be set up as an ErrorDocument on Apache for example. It adds a HEAD request to the fallback server to check if it would serve the requested path. This allows to differenciate legacy requests from actual 404 and serve the most appropriate content:

• fallback to a subdomain for the former,
• custom 404 page for the later