Of course! Below is a sample implementation of the get_requestor_preferred_language() function in PHP. It utilizes the $_SERVER['HTTP_ACCEPT_LANGUAGE'] header to determine the user's preferred language:
```php
function get_requestor_preferred_language() {
  $languages = array(
    'es',
    'de',
    'en',
    'fr',
    'zh',
    'jp'
  );

  if (isset($_SERVER['HTTP_ACCEPT_LANGUAGE'])) {
    $acceptLanguage = strtolower($_SERVER['HTTP_ACCEPT_LANGUAGE']);
    $langs = explode(',', $acceptLanguage);

    foreach ($langs as $lang) {
      $langParts = explode(';', $lang);
      $code = trim($langParts[0]);

      if (in_array($code, $languages)) {
        return $code;
      }
    }
  }

  // fallback to 'en' if no valid language is found
  return 'en';
}

```
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