components/behaviors/CheckboxListBehavior.php
public function beforeUpdate($event)
    {

        if (is_array($this->fields)) {
            foreach ($this->fields as $field) {
                if (is_array($event->sender->{$field})) {
                    $event->sender->{$field} = implode(';', $event->sender->{$field});
                }
            }
        }

    }
public function beforeUpdate($event)
    {

        if (is_array($this->fields)) {
            foreach ($this->fields as $field) {
                if (is_array($event->sender->{$field})) {
                    $event->sender->{$field} = implode(';', $event->sender->{$field});
                }
            }
        }

    }
Change array to string separate by ;
@param $event