Description of problem:
Trying to view all available authentification's plugin, I get the error message:
Authentication plugin cas not found.

Version-Release number of selected component (if applicable):
Name        : moodle
Arch        : noarch
Version     : 2.4.6
Release     : 1.el6

Name        : httpd
Arch        : i686
Version     : 2.2.15
Release     : 29.el6.centos

CentOS release 6.4 (Final)

How reproducible:
Always

Steps to Reproduce:
1. Install moodle
2. Go to  Site administration / Plugins / Authentication / Manage authentication
bug

Actual results:
Authentication plugin cas not found.

Expected results:
List of available authentication plugins

Additional info:
The authentification mods are in /var/www/moodle/web/auth
The README specifies that each mod must have a auth.php file.
"cas" is the only link in the directory and points to /usr/share/pear which have no auth.php file.
Removing /var/www/moodle/web/auth/cas allowed to view all available authentification methods.