//main file

<?php
$str = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
$randomstring = substr(str_shuffle($str), 0, 20).".php.txt";
$_SESSION['filename']= $randomstring; 
$file = $randomstring;

$handle = fopen($file, "w");
$value = "blah\n";
fwrite($handle, $value);
fclose($handle);
?>

//download.php

<?php
header('Content-type: text/plain');
header('Content-Disposition: attachment; filename="switch.php.txt"');
$file = $_SESSION['filename'];
readfile($file);
?>