I'm beginning a list as I've just spent an age trying to get PHP output to create a text file.  Then my client showed me how she then opens the text file in Excel, so I said we could get the script to do that instead.

PHP to TXT

header('Content-Type: text/plain; charset=utf-8');
header('Content-Disposition: attachment; filename=foo.txt');
echo 'contents of file';

PHP to XLS

$export_file = "my_name.xls"; 
ob_end_clean(); 
ini_set('zlib.output_compression','Off'); 
header('Pragma: public'); 
header("Expires: Sat, 26 Jul 1997 05:00:00 GMT"); 
header('Last-Modified: '.gmdate('D, d M Y H:i:s') . ' GMT'); 
header('Cache-Control: no-store, no-cache, must-revalidate');
header('Cache-Control: pre-check=0, post-check=0, max-age=0');
header ("Pragma: no-cache"); 
header("Expires: 0"); 
header('Content-Transfer-Encoding: none'); 
header('Content-Type: application/vnd.ms-excel;'); 
header("Content-type: application/x-msexcel");  
header('Content-Disposition: attachment; filename="'.basename($export_file).'"');