PHPExcel file cannot open file because the file format or file extension is not valid

I'm stuck with this problem, it's not displaying the actual excel file. Please check my code below:

/** Error reporting */
error_reporting(E_ALL);

/** PHPExcel */
require_once 'PHPExcel.php';
include 'PHPExcel/Writer/Excel2007.php';


// Create new PHPExcel object
#echo date('H:i:s') . " Create new PHPExcel object\n";
$objPHPExcel = new PHPExcel();
$excel = new PHPExcel();

$objPHPExcel->getProperties()->setTitle("Payroll");

if(!$result){
die("Error");
}
$col = 0; 
$row = 2; 
while($mrow = mysql_fetch_assoc($result)) { 
$col = 0; 
foreach($mrow as $key=>$value) { 
    $objPHPExcel->getActiveSheet()->setCellValueByColumnAndRow($col, $row, $value); 
    $col++; 
} 
$row++;  
} 

 // Set active sheet index to the first sheet, so Excel opens this as the first sheet
 $objPHPExcel->setActiveSheetIndex(0)
        ->setCellValue('A1', 'Scholar Id')
        ->setCellValue('B1', 'Lastname')
        ->setCellValue('C1', 'Middlename')
        ->setCellValue('D1', 'Firstname')
        ->setCellValue('E1', 'Barangay')
        ->setCellValue('F1', 'Level')
        ->setCellValue('G1', 'Allowance')
        ->setCellValue('H1', 'Has claimed?');
        $objPHPExcel->getActiveSheet()->getStyle('A1:H1')->getFont()->setBold(true);
        $objPHPExcel->getActiveSheet()->getColumnDimension('A')->setWidth(12);
        $objPHPExcel->getActiveSheet()->getColumnDimension('B')->setWidth(18);
        $objPHPExcel->getActiveSheet()->getColumnDimension('C')->setWidth(18);
        $objPHPExcel->getActiveSheet()->getColumnDimension('D')->setWidth(18);
        $objPHPExcel->getActiveSheet()->getColumnDimension('E')->setWidth(18);
        $objPHPExcel->getActiveSheet()->getColumnDimension('F')->setWidth(12);
$objPHPExcel->getActiveSheet()->getColumnDimension('G')->setWidth(12);
$objPHPExcel->getActiveSheet()->getColumnDimension('H')->setWidth(14);
$objPHPExcel->getActiveSheet()->getStyle('A1:H1')->getAlignment()- >setHorizontal(PHPExcel_Style_Alignment::HORIZONTAL_CENTER);
$objPHPExcel->getActiveSheet()->setShowGridlines(true);

$objPHPExcel->getActiveSheet()->getStyle('A1:H1')->applyFromArray(
array(
    'fill' => array(
        'type' => PHPExcel_Style_Fill::FILL_SOLID,
        'color' => array('rgb' => 'FFFF00')
    )
)
); 


// Save Excel 2007 file
echo date('H:i:s') . " Write to Excel2007 format\n";
#$objWriter = PHPExcel_IOFactory::createWriter($objPHPExcel, 'Excel2007');
$objWriter = new PHPExcel_Writer_Excel2007($objPHPExcel);
header('Content-Type: application/vnd.ms-excel');
header('Content-Disposition: attachment;filename="payroll.xlsx"');
header('Cache-Control: max-age=0'); 
$writer->save('php://output');

I got it working now! Thanks to this phpexcel to download

I changed the code to this:

// Save Excel 2007 file
#echo date('H:i:s') . " Write to Excel2007 format\n";
$objWriter = PHPExcel_IOFactory::createWriter($objPHPExcel, 'Excel2007');
ob_end_clean();
// We'll be outputting an excel file
header('Content-type: application/vnd.ms-excel');
header('Content-Disposition: attachment; filename="payroll.xlsx"');
$objWriter->save('php://output');

I think this line:

ob_end_clean();

solved my problem.