This iterates through a variable number of columns of a particular row, which in this case is the 1st row:
$rownumber = 1;
$row = $this->objPHPExcel->getActiveSheet()->getRowIterator($rownumber)->current();
$cellIterator = $row->getCellIterator();
$cellIterator->setIterateOnlyExistingCells(false);
foreach ($cellIterator as $cell) {
$cell->getStyle()->getFont()->setBold(true);
}