PHP创建Excel文件，变音符问题

Hello I am creating a Excel file with PHP. I am setting the header, before I print out some data:

header('Content-type: application/vnd.ms-excel; charset=utf-8');

And then I am inserting something like:

echo '<table><tr><td>täääst</td></tr></table>';

My problem is that the ä isn't shown right in the excel file.

I am also having an error, when I open the file that says:

The file you are trying to open, 'filename.xls', is in a different format than specified by the file extension. Verify that the file is not corrupted and is from a trusted source before opening the file. Do you want to open the file now?

Can you help me please? Thank you in advance.