<?php
$valid_chars = str_split('0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz');
$length = 40;

header('Content-Type: text/plain');
for($i=0;$i<$length;$i++)
	echo $valid_chars[array_rand($valid_chars)];
echo PHP_EOL;
?>