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
replaces the 13.
It is also limited to ASCII characters between 32 and 127 (included).
Other characters will not be encrypted.