MyExcel.php

<?php
class MyExcel {

    private static $format = '';

    public static function setFormat(sfEvent $event) {
        self::$format = $event['format'];
    }

    public static function setContent(sfEvent $event, $response) {
        if (self::$format == 'xlsx') {
            // Include PHPExcel and its autoloader
            include(sfConfig::get('sf_lib_dir').'/vendor/phpexcel/Classes/PHPExcel.php');

            // write response to an xml file (in prod, use unique filenames should be used)
            $temporary_file_name = sfConfig::get('sf_cache_dir').'/excel.xml';
            file_put_contents($temporary_file_name, $response);

            // load the file
            $excel_reader = new PHPExcel_Reader_Excel2003XML();
            $excel_sheet = $excel_reader->load($temporary_file_name);

            // output the file
            $excel_writer = new PHPExcel_Writer_Excel2007($excel_sheet);
            $excel_writer->save('php://output');
        }
    }

}