Hi benoit1980, First of all, sorry for "late" answer It seems that You don't have namespace for ActiveRecord class
use yii\db\ActiveRecord;
If You won't specify this, PHP will look for that class in current namespace, like You see in throwed exception, 'app\models'.
The other way is to override ActiveRecord class in 'app\models'
<?php
namespace app\models;
use yii\db\ActiveRecord as BaseActiveRecord;
class ActiveRecord extends BaseActiveRecord
{
// ...
}
Tip: IDE like NetBeans have auto detecting for namespaces (Ctrl+Shif+I), which can save alot of time