OR statements in metatags...

metatag_example.php

<?php

/**
 * Implements hook_metatags_attachments_alter().
 *
 * This function allows you to define fallback tokens in case a field is empty.
 *
 * If all fallback values are empty, the tag will be empty.
 *
 * Example: [node:field_image:medium]||[node:field_legacy_image:medium]||/fallback.png
 */
function example_metatags_attachments_alter(array &$metatag_attachments) {
  if (!empty($metatag_attachments['#attached']['html_head'])) {
    $url = \Drupal::request()->getSchemeAndHttpHost();
    $insecure_url = str_replace('https://', 'http://', $url);
    foreach ($metatag_attachments['#attached']['html_head'] as &$attachment) {
      if (isset($attachment[0]['#attributes']['content'])) {
        $value = &$attachment[0]['#attributes']['content'];
      }
      elseif (isset($attachment[0]['#attributes']['href'])) {
        $value = &$attachment[0]['#attributes']['href'];
      }
      if (isset($value) && strpos($value, '||') !== FALSE) {
        $new_value = FALSE;
        foreach (explode('||', $value) as $option) {
          // We have to check against the URL because metatag will prepend our
          // special tag value with the site URL if the tag plugin uses
          // absolute URLs.
          if (!empty($option) && $option !== $url && $option !== $insecure_url) {
            // Make relative URLs absolute.
            if (strpos($option, '/') === 0) {
              $option = rtrim($url, '/') . $option;
            }
            $new_value = trim($option);
            break;
          }
        }
        $value = $new_value ?: '';
      }
    }
  }
}

We had to add an is_string check to $values because sometimes $value is returned as an array and strpos can only handle strings.

-      if (isset($value) && strpos($value, '||') !== FALSE) {
+      if (isset($value) && is_string($value) && strpos($value, '||') !== FALSE) {

Credit goes to @banoodle

PHPstan didn't like the above. Changing it to the following works and passes.

-    if (isset($value) && is_string($value) && strpos($value, '||') !== FALSE) {
+   if (is_string($value) && strpos($value, '||') !== FALSE) {