I've used this _very_ simple code. The file works just fine after being downloaded. I also note that your 2nd last line appears to be incorrect. You should have an equals [=] symbol after filename, not a colon[:]
<?php
header('Content-type: application/vnd.openxmlformats-officedocument.spreadsheetml.sheet');
header('Content-Disposition: attachment; filename=download.xlsx');
readfile('aria.xlsx');
?>
I note that it still functions when we comment-out the first line:
<?php
header('Content-Disposition: attachment; filename=download.xlsx');
readfile('aria.xlsx');
?>
If you don't have it already, you should _really_ consider downloading the *enhanced* version of the php manual. It's the same as the regular version, albeit with the user contributions that you get on the website. It's about 6 or 7 meg of user examples and notes. It's light-years in front of the standard version.
You can get it here:
Select mirror for php_enhanced_en.chm[
^]