code: function MsExcel_SheetIsIt($FileName) {
  		$this->MsExcel_SheetInit();
  		foreach($this->MsExcel_Sheets as $o) {
  			if ($FileName=='xl/'.$o->file) return true;
  		}
  		return false;
  	}
docs: Check if the file name is a subfile corresponding to a sheet.