Manual:$wgPasswordSalt

Details
For security, the user password hashes include "salt" to make it more difficult for someone who somehow gets hold of the hashes to crack them all at once.

For compatibility with old (pre 1.1.0) installations set this to false.

To reset the password of users when this option is set use the following query:

UPDATE user SET user_password=md5(concat(user_id,'-',md5('mynewpassword'))) where user_name ='WikiSysop' ;

user_id is the userid of the user, mynewpassword has to be your new password and WikiSysop could be another user.

When you want to let users login with the username and password of the wiki database use the following php code:

$passwd="secret";

$id=1;

$userpasswdsalt=md5($id."-".md5($passwd));