This is very useful to prevent spam robots collecting email addresses from your site, included is a method to add mailto links to the text being generated.
A basic example to encrypt an email address
// encrypt 'firstname.lastname@example.org'
// with offset 8
$c = new HTML_Crypt('email@example.com', 8);
// add <a href="mailto:"></a> around this email
The "encryption" function is basically works like ROT13, with the difference that the $offset setting replaces the 13. It is also limited to ASCII characters between 32 and 127 (included). Other characters will not be encrypted.