Display SKU in order page of magento admin panel

To display SKU in order page of magento admin panel :


public function callback_skus($value, $row, $column, $isExport) {
$_items = Mage::getModel('sales/order')->loadByIncrementId($value)->getAllItems();

$skus_arr = array();
foreach ($_items as $item) {
array_push($skus_arr, $item->getSku());
}
$skus_arr = array_unique($skus_arr);

return implode(”
“,$skus_arr);
}


$this->addColumn('increment_id', array(
'header' => Mage::helper('sales')->__('SKU\'s'),
'index' => 'increment_id',
'frame_callback' => array($this, 'callback_skus')
));