public function update_header_js($js) {
        // Update the names of the the configFn variables.
        // The PHP json_encode function cannot handle literal names so we have to wrap
        // them in @ and then replace them with literals of the same function name.
        foreach ($this->jsconfigfunctions as $functionname => $function) {
            $js = str_replace('"@' . $functionname . '@"', $functionname, $js);
        }
        return $js;
    }