/** * Unset an instance of this class * * @access public * @param PHPExcel $workbook Injected workbook identifying the instance to unset */ public static function unsetInstance(PHPExcel $workbook = NULL) { if ($workbook !== NULL) { if (isset(self::$_workbookSets[$workbook->getID()])) { unset(self::$_workbookSets[$workbook->getID()]); } } }