Pantheon: Redirect via JSON feed in private file section

redirect-from-json.php

// PHP snippet included in wp-config.php (or settings.php).
// Including from that location in a separate file is OK as well.

<?php
// Remove any leading "www." from the host name.
$redirect_host = str_replace('www.', '', $_SERVER['HTTP_HOST']);
$redirect_path = strtolower(rtrim($_SERVER['REQUEST_URI']));
if (strlen($redirect_path) > 2) {
  $redirect_path = rtrim($redirect_path, '/');
}
$redirect_search = strtolower(($redirect_host . $redirect_path));
$redirects_json = dirname(__FILE__) . '/files/private/redirects/redirects.json';
if (file_exists($redirects_json)) {
  $redirect_patterns = json_decode(file_get_contents($redirects_json), TRUE);

  // Exact match redirects
  if (in_array($redirect_search, array_keys($redirect_patterns['singles']))) {
    $redirect_to = $redirect_patterns['singles'][$redirect_search];
    header('HTTP/1.0 301 Moved Permanently');
    header('Location: ' . $redirect_to);
    exit();
  }

  // Domain based redirects.
  if (in_array($redirect_host, array_keys($redirect_patterns['domains']))) {
    $redirect_to = $redirect_patterns['domains'][$redirect_host];
    header('HTTP/1.0 301 Moved Permanently');
    header('Location: ' . $redirect_to);
    exit();
  }
}

redirects.json

{
  "singles": {
    "example.com/page1": "https://example.net/new-page-1",
    "example.com/page2": "https://example.net/new-page-2"
  },
  "domains": {
    "example.org": "https://example.net/all-from-example-org",
    "example.foo": "https://example.net/redirect-from-example-foo"
  }
}

Thank you to David Lanier (via Pantheon Slack channel) for this snippet.
You might want to contact him for more... [email protected]