Preprocess field and replace HTTP with HTTPs for Sched.org schedules

MYMODULE.module.php

<?php

/**
 * Implements hook_preprocess_field().
 */
function HOOK_preprocess_field(&$variables) {
  if ('field_FIELDNAME' == $variables['element']['#field_name'] && isset($GLOBALS['is_https']) && $GLOBALS['is_https']) {
    if (!empty($variables['items'])) {
      foreach (element_children($variables['items']) as $delta) {
        if (!empty($variables['items'][$delta]['#markup'])) {
          $matches = array();
          if (preg_match_all('/((https?:\/\/)?([a-z0-9\.-]+)(.sched.org\/js\/embed.js))/i', $variables['items'][$delta]['#markup'], $matches)) {
            if (!empty($matches[0])) {
              $replacement = str_replace('http://', 'https://', $matches[0]);
              $variables['items'][$delta]['#markup'] = str_replace($matches[0], $replacement, $variables['items'][$delta]['#markup']);
            }
          }
        }
      }
    }
  }
}