protected function registerCallbacks()
    {
        self::$_callbacks = $this->xslCallbacks;
        $this->xsltProcessor->registerPHPFunctions(get_class($this) . '::invokeCallback');
    }
protected function registerCallbacks()
    {
        self::$_callbacks = $this->xslCallbacks;
        $this->xsltProcessor->registerPHPFunctions(get_class($this) . '::invokeCallback');
    }
protected function registerCallbacks ( ) { self :: $ _callbacks = $ this -> xslCallbacks ; $ this -> xsltProcessor -> registerPHPFunctions ( get_class ( $ this ) . '::invokeCallback' ) ; }
register all callback instances

Workaround for limitation of XSLTProcessor::registerPHPFunctions()
callback instance in static variable to have data available when
php:function callback calls the static method as
XSLTProcessor::registerPHPFunctions() does not support non-static
methods nor anonymous functions directly.
register all callback instances