headers already sent

[matthew perry <mwperry@xxxxxxxxxxx>]

When I insert the following header:

     header ("Content-type: application/vnd.ms-excel");
     header ("Content-Disposition: attachment" );

I get the messages:

*Warning*: Cannot modify header information - headers already sent by 
(output started at C:\Program Files\Apache Group\Apache2\htdocs\trial.php:2) in 
*C:\Program Files\Apache Group\Apache2\htdocs\trial.php* on line 
*3*

*Warning*: Cannot modify header information - headers 
already sent by (output started at C:\Program Files\Apache 
Group\Apache2\htdocs\trial.php:2) in *C:\Program Files\Apache 
Group\Apache2\htdocs\trial.php* on line *4


*I get similar messages when I use my web host.  How do I use these headers correctly?

Matt