As per my above comment, you can only force to have a download option. For this you can set headers in this way -
header("Content-Type: application/vnd.openxmlformats-officedocument.spreadsheetml.sheet");
header("Content-Disposition: attachment;filename=\"filename.xlsx\"");
header("Cache-Control: max-age=0");
Reference - PHP Excel Reader
For more options you can also check the cheat sheet - Cheat Sheet
Although the best way to read here - Codeplex
EDIT
Do something like this -
$excel = new PHPExcel();
header('Content-Type: application/vnd.openxmlformats-officedocument.spreadsheetml.sheet');
header('Content-Disposition: attachment;filename="your_name.xls"');
header('Cache-Control: max-age=0');
// Do your stuff here
$writer = PHPExcel_IOFactory::createWriter($excel, 'Excel5');
// This line will force the file to download
$writer->save('php://output');