usermod example - Add a existing user to existing group

usermod -a -G sysadmins tony

PAM authentication in PHP

sudo aptitude install php5-auth-pam
sudo cp /usr/share/doc/php5-auth-pam/examples/php /etc/pam.d/php

PHP example:

$error = 0;
if(pam_auth("zappedy", "XXXX", &$error))
 echo "bien";
else
 echo "mal $error";

Ref:
http://www.dokuwiki.org/auth:pam