I think maybe you're looking for the PHP to make these download, not the HTML!
First, you need to get the headers right for your download:
header('Content-type: text/plain');
header('Content-Disposition: attachment; filename="download.txt"');
Obviously you'll need to substitute the example with the correct mime type and extension for an iCal file.
Then, just use PHP to echo or print out the lines of the file as you need.
