Add require($phpbb_root_path . 'includes/CAS.' . $phpEx);
Find (isset($_POST['login']))
Replace it with:
if(!$admin && CAS_ENABLE){ 
// initialize phpCAS 
phpCAS::client(CAS_VERSION_2_0, CAS_SERVER_HOSTNAME, CAS_SERVER_PORT, CAS_SERVER_APP_NAME); 
phpCAS::setNoCasServerValidation(); 
// force CAS authentication 
phpCAS::forceAuthentication(); 
} 
if (isset($_POST['login']) || (!$admin && CAS_ENABLE) )
Find $result = $auth->login($username, $password, $autologin, $viewonline, $admin);
Add the following code before it:
if(!$admin && CAS_ENABLE){ 
$username = phpCAS::getUser(); 
$password = ''; 
}

Add the following function:
function login_cas($username){ 
global $db; 
$sql = 'SELECT user_id, username, user_password, user_passchg, user_pass_convert, user_email, user_type, user_login_attempts FROM ' . USERS_TABLE 
. " WHERE username_clean = '" . $username . "'"; 
$result = $db->sql_query($sql); 
$row = $db->sql_fetchrow($result); 
$db->sql_freeresult($result); 

if (!$row) { 
return array( 
'status' => LOGIN_ERROR_USERNAME, 
'error_msg' => 'LOGIN_ERROR_USERNAME', 
'user_row' => array('user_id' => ANONYMOUS), 
); 
} 

if ($row['user_type'] == USER_INACTIVE || $row['user_type'] == USER_IGNORE) { 
return array( 
'status' => LOGIN_ERROR_ACTIVE, 
'error_msg' => 'ACTIVE_ERROR', 
'user_row' => $row, 
); 
} 

// Successful login... set user_login_attempts to zero... 
return array( 
'status' => LOGIN_SUCCESS, 
'error_msg' => false, 
'user_row' => $row, 
); 
}
Find $login = $method($username, $password);
Replace it with:
if(!$admin && CAS_ENABLE)    
$login = $this->login_cas($username);    
else    
$login = $method($username, $password);

Add the following constant value:
define('CAS_ENABLE', true); 
define('CAS_SERVER_HOSTNAME', 'localhost'); 
define('CAS_SERVER_PORT', 8443); 
define('CAS_SERVER_APP_NAME', 'cas');

[phpBB Debug] PHP Notice: in file /includes/CAS/client.php on line 529: session_destroy() [function.session-destroy]: Trying to destroy uninitialized session
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phpCAS ${phpcas.version} using server https://localhost:8443/cas/ (CAS 2.0)