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I am using horde 4.0.11, IMP 5.0.14 on debian squeeze.  Horde 
authentication is done by SQL.
I also make use of horde username hook.  IMP login is done by configuring
the backends.php.

I would like to use Passwd to update both passwords of horde and email
backend (running vpopmail).  Therefore, horde (user) and vpopmail (
user at example.com) have to be updated in the same time.  It seems me that
hook of Passwd must be used.

My method is to strip the domain from $userid in function username and then
add the domain back if the backend is 'expect'.

However, the screen just turn white whenever I try to reset a user's
password.  After some investigation, I believe the problem is from the
statement:         foreach ($driver->_params['drivers'] as $backend =>
$config) {

My syslog capture the php error as:
Nov 22 20:44:05 mail apache2: PHP Fatal error:  Cannot access 
protected property Passwd_Driver_Composite::$_params in 
/var/www/html/horde4/passwd/config/hooks.php on line 36
Nov 22 20:44:05 mail apache2: PHP Fatal error:  Cannot access 
protected property Passwd_Driver_Composite::$_params in 
/var/www/html/horde4/passwd/config/hooks.php on line 36
Nov 22 20:44:05 mail apache2: PHP Fatal error:  Cannot access 
protected property Passwd_Driver_Composite::$_params in 
/var/www/html/horde4/passwd/config/hooks.php on line 36

I have attached the relevant configuration files including horde hook, 
passwd backend and passwd hook in attachment.