via a Php Script?
Apparently you can just create an HTML table, and save the file with the *.xls extension, and voila... you have an Excel Spreadsheet. I personally have never tried it, but I believe weedpacket has mentioned it in the past.
SELECT ....... INTO OUTFILE .. etc will output a csv, which of course loads straight into Excel. All depends which ways suits you best.
http://dev.mysql.com/doc/mysql/en/SELECT.html
You can create an HTML document containing Excel-specific CSS styles (these are ignored by browsers but used by Excel).
Excel can read this in and will use them to format the cells.
Mark
There's also a dandy little PEAR package that'll give you quite a bit of control over the spreadsheet formatting.