custom user field value in php

Using a page node, i have a template html which is calling a php file.I'm trying to pass the customfield value through as a param.. It is not working as i would have hoped.. Any ideas?

Template HTML:
<xen:callback class="Stamps_Installer" method="getHtml" params="{$visitor.customFields.Stamp}"></xen:callback>

PHP File:
class Stamps_Installer
    public static function getHtml($params)
      echo 'test';
    echo $params;