Fomatting excel before downloading using php

I am able to format a excel column to 2 decimal places using php
with the help of number_format function.
I am also see the formatted values in browser.
But when i download that as a excel file,decimal values
are getting rounded off automatically eg: 8.10 is shown as 8.1 .
I need to show only 8.1 in excel without ruounding off.
Can anybody suggest on this?