I'm not sure exactly what you need, but it is possible to have a mobile platform (iOS, Android, etc) download a file. With some minor quirks, mobile browsers behave just like their desktop counterparts.
You can provide the user with a direct link like http://example.com/somefile.zip
If you need to do some processing via PHP first, it can look like
<?php
// can I download this file
if ( $itsoktodownload == true ) {
// some code here
header("Location: http://example.com/somefile.zip);
exit;
}
?>
Basically, that code snippet is just redirecting the user to the url of the downloaded file.
If you need something more specific, like you need to force it to download (instead of attempting to "open"), or you need to control the headers sent first, or you are trying to hide the real location of the file (so people can't share the link around), there are many download scripts available via google.
You could ask a more specific question once you've gotten into it a bit.
header('Content-Disposition: attachment; filename="downloaded.mp3"');