Different robots.txt files for HTTP and HTTPS

.htaccess

RewriteEngine On

#This will have the file processed by robots.php instead of a regular static file.
RewriteRule ^robots.txt$ /robots.php [L]

robots.php

<?php

header("Content-Type: text/plain; charset=utf-8");
$protocol = $_SERVER['HTTPS'] ? 'HTTPS' : 'HTTP';
echo file_get_contents(dirname(__FILE__).'/robots.txt.'.strtolower($protocol));

robots.txt.http

User-agent: *
Disallow:

robots.txt.https

User-agent: *
Disallow: /

This tells robot crawlers to read every HTTP path and none HTTPS.