gistfile1.php

public function getCsv()
{
    $csv = '';
    $this->_isExport = true;
    $this->_prepareGrid();

    $sql = (string)$this->getCollection()->getSelect()->__toString();

    $db       = Mage::getResourceSingleton('core/resource')->getReadConnection();
    $result   = $db->query($sql);
    $per_page = 100;
    $pages    = ceil($result->rowCount()/$per_page);

    $data = array();
    foreach ($this->_columns as $column) {
        if (!$column->getIsSystem()) {
            $data[] = '"'.$column->getExportHeader().'"';
        }
    }
    $csv.= implode(',', $data)."\n";

    for ($page=1; $page<=$pages; $page++) {
        $this->getCollection()->setPage($page,$per_page);
        $this->getCollection()->load();
        $this->_afterLoadCollection();

        foreach($this->getCollection() as $item) {
            $data = array();
            foreach ($this->_columns as $column) {
                if (!$column->getIsSystem()) {
                    $data[] = '"' . str_replace(array('"', '\\'), array('""', '\\\\'),
                        $column->getRowFieldExport($item)) . '"';
                }
            }
            $csv.= implode(',', $data)."\n";
        }

    }

    if ($this->getCountTotals())
    {
        $data = array();
        foreach ($this->_columns as $column) {
            if (!$column->getIsSystem()) {
                $data[] = '"' . str_replace(array('"', '\\'), array('""', '\\\\'),
                    $column->getRowFieldExport($this->getTotals())) . '"';
            }
        }
        $csv.= implode(',', $data)."\n";
    }

    return $csv;
}