Description:
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One of the characters used in MIME Base64 is '+', which is a special character in URL query strings.

A common mistake is to use the result of base64_encode directly in a URL.  Example:

http://docs.joomla.org/index.php?title=How_do_you_redirect_users_after_a_successful_login%3F&oldid=36843

I even found some instances of this mistake in Joomla itself.  I noticed it while troubleshooting an issue where com_login would send the user to a useless page saying "Welcome to the registered user area of our site."

I believe the documentation for base64_encode should warn about this.  See the test script for examples.

Test script:
---------------
<?php

$return = "http://localhost/~me";

// WRONG.  This will be decoded server-side as "http://localhost/[Y"
$url = "login?return=" . base64_encode($return);

// Base64 must be URL-encoded when used in a URL, because it can contain '+'
// characters.
$url = "login?return=" . urlencode(base64_encode($return));

?>