public function isTrustedPhpFunction($function_name, $compiler)
    {
        if (isset($this->php_functions) && (empty($this->php_functions) || in_array($function_name, $this->php_functions))) {
            return true;
        }

        $compiler->trigger_template_error("PHP function '{$function_name}' not allowed by security setting");

        return false; // should not, but who knows what happens to the compiler in the future?
    }