Fully working example

FullExample.php

require_once("PHPExcel/PHPExcel.php");
$phpExcel = new PHPExcel();

$styleArray = array(
	'font' => array(
		'bold' => true,
	)
);

//Get the active sheet and assign to a variable
$foo = $phpExcel->getActiveSheet();

//add column headers, set the title and make the text bold
$foo->setCellValue("A1", "Foo1")
	->setCellValue("B1", "Foo2")
	->setCellValue("C1", "Foo3")
	->setCellValue("D1", "Foo3")
	->setTitle("Foo")
	->getStyle("A1:D1")->applyFromArray($styleArray);

//Create a new sheet
$bar = $phpExcel->createSheet();
$bar->setCellValue("A1", "Bar1")
	->setCellValue("B1", "Bar2")
	->setCellValue("C1", "Bar3")
	->setCellValue("D1", "Bar3")
	->setTitle("Bar")
	->getStyle("A1:D1")->applyFromArray($styleArray);

//When in loops you always need to use a counter to ensure data goes into the next row.
for ($rowCounter = 2; $rowCounter < 20; $rowCounter++) {

	$foo->setCellValue("A$rowCounter", "Row" . ($rowCounter - 2))
		->setCellValue("B$rowCounter", $rowCounter * 2)
		->setCellValue("C$rowCounter", $rowCounter / 2)
		->setCellValue("D$rowCounter", "=B$rowCounter+C$rowCounter");

	$bar->setCellValue("A$rowCounter", "Row" . ($rowCounter - 2))
		->setCellValue("B$rowCounter", ($rowCounter % 2) ? "Type 1" : "Type2")
		->setCellValue("C$rowCounter", str_repeat("foo ", rand(5, 10)))
		->setCellValue("D$rowCounter", str_repeat("% ", rand(20, 50)));
}

//Merge the first two columns of the next row and sum columns C & D.
$foo->mergeCells("A$rowCounter:B$rowCounter");
$foo->setCellValue("A$rowCounter", "Total")
	->setCellValue("C$rowCounter", "=SUM(C2:C" . ($rowCounter -1) . ")")
	->setCellValue("D$rowCounter", "=SUM(D2:D" . ($rowCounter -1) . ")");

//Set the text alignment to right for the total cell.
$foo->getStyle("A$rowCounter")->getAlignment()->setHorizontal(PHPExcel_Style_Alignment::HORIZONTAL_RIGHT);

//Set the column widths
$foo->getColumnDimension("A")->setWidth(40);
$foo->getColumnDimension("B")->setWidth(20);
$foo->getColumnDimension("C")->setWidth(20);
$foo->getColumnDimension("D")->setWidth(20);

$bar->getColumnDimension("A")->setAutoSize(true);
$bar->getColumnDimension("B")->setAutoSize(true);
$bar->getColumnDimension("C")->setAutoSize(true);
$bar->getColumnDimension("D")->setWidth(40);

//Wrap long fields
$bar->getStyle("D1:D20")->getAlignment()->setWrapText(true);

//Set the active sheet to the first sheet before outputting. This is only needed if you want to ensure the file is opened on the first sheet.
$phpExcel->setActiveSheetIndex(0);

//Output the generated excel file so that the user can save or open the file.
header("Content-Type: application/vnd.ms-excel");
header("Content-Disposition: attachment; filename=\"example-excel-file.xls\"");
header("Cache-Control: max-age=0");

$objWriter = PHPExcel_IOFactory::createWriter($phpExcel, "Excel5");
$objWriter->save("php://output");
exit;