too long

Question

i am using my sql and php code, user table have password column as char(32) and salt column as char(64)

Now from php code i am trying to authenticate using below code

$this->_authAdapter = new Zend_Auth_Adapter_DbTable(
    $db,
    Engine_Api::_()->getItemTable('user')->info('name'),
    'user_id',
    'password',
    "MD5(CONCAT('".$salt."', ?, salt)))"

  );

but it gives me following error: The supplied parameters to Zend_Auth_Adapter_DbTable failed to produce a valid sql statement, please check table and column names for validity.

any suggestion??