PEAR::Auth_RADIUS provides wrapper-classes for the RADIUS PECL.
There are different Classes for the different authentication methods.
If you are using CHAP-MD5 or MS-CHAP you need also the Crypt_CHAP package.
If you are using MS-CHAP you need also the mhash and mcrypt extension.

WWW: http://pear.php.net/package/Auth_RADIUS/