Function to encrypt and unencrypt data?

Is there a function for PHP that can encrypt and unencrypt data?

base64_encode($string);

base64_decode($string);

It is not really an encryption ... Base64 is used to encode binary data before sending it over some channel which does provide transporting for 8-bit encoded characters ...

For encryption please see e.g. Mcrypt functions (http://www.php.net/manual/en/ref.mcrypt.php).