// set the font style for the entire workbook
$phpExcel->getDefaultStyle()->getFont()
    ->setName('Arial')
    ->setSize(14)
    ->setBold(true);

// custom style with different font than the global one
$styleArray = array(
    'font'  => array(
        'bold' => true,
        'color' => array('rgb' => 'FF0000'),
        'size'  => 15,
        'name' => 'Verdana'
    ));

// create some cells
$phpExcel->getActiveSheet()->getCell('A1')->setValue('Hello world');
$phpExcel->getActiveSheet()->getCell('A2')->setValue('Hello again');
$phpExcel->getActiveSheet()->getCell('A3')->setValue('Goodbye');

// apply custom style to single cell
$phpExcel->getActiveSheet()->getStyle('A3')->applyFromArray($styleArray);