PHP error in hook element

I'm trying to create a button that downloads a text file that is generated from a php script.

So far I have:

<?php //Generate text file on the fly
 
header("Content-type: text/plain");
header("Content-Disposition: attachment; filename=savethis.txt");
 
// do your Db stuff here to get the content into $content
print "This is some text...";
print $content;
?> 

If I use a hook to show this after the content on a page I get the following error:

Warning: Cannot modify header information - headers already sent by (output started at /home/onecards/public_html/wp-includes/class.wp-styles.php:290) in /home/onecards/public_html/wp-content/plugins/gp-premium/elements/class-hooks.php(196) : eval()'d code on line 3

Warning: Cannot modify header information - headers already sent by (output started at /home/onecards/public_html/wp-includes/class.wp-styles.php:290) in /home/onecards/public_html/wp-content/plugins/gp-premium/elements/class-hooks.php(196) : eval()'d code on line 4

If I put the code into a .php file and call that with a href link it works.

Thanks!