I do not think either PHPExcel making change, but rather Excel.
Two solutions: set these values to a string (by setCellValueExplicit) or achieve with Excel a whole little workbook with a value formatted as you want, watch how Excel notes this format internally (*) and use it with PHPExcel.
(*) You can determine this by directly reading the xml files or by loading this file with PHPExcel and then asking to show you the format of the affected cell.
Two solutions: set these values to a string (by setCellValueExplicit) or achieve with Excel a whole little workbook with a value formatted as you want, watch how Excel notes this format internally (*) and use it with PHPExcel.
(*) You can determine this by directly reading the xml files or by loading this file with PHPExcel and then asking to show you the format of the affected cell.