$objPHPExcel->getSheetByName('Worksheet 1')
->setSheetState(PHPExcel_Worksheet::SHEETSTATE_HIDDEN);
or
$objPHPExcel->getSheetByName('Worksheet 1')
->setSheetState(PHPExcel_Worksheet::SHEETSTATE_VERYHIDDEN);
EDIT
You can use Excel's Format/Sheet/Hide to hide an entire worksheet. This sets the worksheet's visible property to xlSheetHidden. But unless you password-protect the workbook structure, anyone can select Format/Sheet/Unhide to see the hidden sheet.
If you use Excel 97 or later, then you can "very hide" a sheet:
- Press Alt-F11 to display the Visual Basic Editor
- in the Project window, double-click Microsoft Excel Objects and select the sheet to hide.
- Press F4 to display the Property box
- Set the sheet's Visible property to xlSheetVeryHidden.
Now the sheet is no longer accessible via Format/Sheet/Unhide
This is what PHPExcel does rather more simply when you set SheetState to VERYHIDDEN