Error Gtm.php

protected function _getDataLayer() {
        // Initialise our data source.
        $data = array();

        // Get transaction and visitor data, if desired.
        if (Mage::helper('googletagmanager')->isDataLayerTransactionsEnabled())
            $data = $data + $this->_getTransactionData();
        if (Mage::helper('googletagmanager')->isDataLayerVisitorsEnabled())
            $data = $data + $this->_getVisitorData();

        // Enable modules to add custom data to the data layer
        $data_layer = new Varien_Object();
        $data_layer->setData($data);
        Mage::dispatchEvent('googletagmanager_get_datalayer', array('data_layer' => $data_layer));
        $data = $data_layer->getData();

        // Generate the data layer JavaScript.
        if (!empty($data))
            return "<script>dataLayer = [" . json_encode($data) . "];</script>\n\n";
        else
            return '';
    }