Problems with replacing CiviCRM tokens in API - php

Question

I'm currently developing a CiviCRM extension, where I need to replace CiviCRM-Tokens (used in pdf and mailing generation) in html code.
I did a little bit of research in the core files and tried to recreate the behaviour in the PDFLetterCommon.php (/civicrm/CRM/Contact/Form/Task/PDFLetterCommon.php) where it replaces the tokens in the postProcess function.
Here is the original CiviCRM Code:
list($formValues, $categories, $html_message, $messageToken, $returnProperties) = self::processMessageTemplate($form);
$skipOnHold = isset($form->skipOnHold) ? $form->skipOnHold : FALSE;
$skipDeceased = isset($form->skipDeceased) ? $form->skipDeceased : TRUE;
foreach ($form->_contactIds as $item => $contactId) {
$params = array('contact_id' => $contactId);
list($contact) = CRM_Utils_Token::getTokenDetails($params,
$returnProperties,
$skipOnHold,
$skipDeceased,
NULL,
$messageToken,
'CRM_Contact_Form_Task_PDFLetterCommon'
);
...
}
And here is my version for testing:
(this code is located inside an api function in my extension)
$messageToken = CRM_Utils_Token::getTokens($params["html"]);
$returnProperties = array();
if (isset($messageToken['contact'])) {
foreach ($messageToken['contact'] as $key => $value) {
$returnProperties[$value] = 1;
}
}
$skipOnHold = FALSE;
$skipDeceased = TRUE;
$tokenParams = array("contact_id" => 67450);
list($contact) = CRM_Utils_Token::getTokenDetails($tokenParams,
$returnProperties,
$skipOnHold,
$skipDeceased,
NULL,
$messageToken,
'CRM_Contact_Form_Task_PDFLetterCommon'
);
I'm using the default values for $skipOnHold (false) and $skipDeceased (true) and also just passing one (existing) user id into the $params array ($tokenParams in my code).
Here is my problem:
My $messageToken and $returnProperties variables are being filled correctly via CiviCRM's core functions but when I pass them all into CRM_Utils_Token::getTokenDetails() the returned $contact variable holds an empty array.
I'm really out of ideas, I've been looking into CRM/Utils/Token.php where getTokenDetails() is located, but have been unsuccessful in finding the problem with my code.
Thanks in advance for any help!