As noted in the str_rot13 docs, some servers don't provide the str_rot13() function. However, the presence of strtr makes it easy to build your own facsimile thereof:
if (!function_exists('str_rot13')) {
function str_rot13($str) {
$from = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
$to = 'nopqrstuvwxyzabcdefghijklmNOPQRSTUVWXYZABCDEFGHIJKLM';
return strtr($str, $from, $to);
}
}
This is suitable for very light "encryption" such as hiding email addressess from spambots (then unscrambling them in a mail class, for example).
$mail_to=str_rot13("$mail_to");