if you want to align only this cells, you can do something like this:
$style = array(
'alignment' => array(
'horizontal' => PHPExcel_Style_Alignment::HORIZONTAL_CENTER,
)
);
$sheet->getStyle("A1:B1")->applyFromArray($style);
But, if you want to apply this style to all cells, try this:
$style = array(
'alignment' => array(
'horizontal' => PHPExcel_Style_Alignment::HORIZONTAL_CENTER,
)
);
$sheet->getDefaultStyle()->applyFromArray($style);