$array = [0 => 'red', 1 => 'green', 2 => 'blue'];
$array = implode($array,',');
// Array now looks like:
$array = 'red,green,blue';
// Use it like this
// I'm using array_keys() here because I want 0,1,2 instead of red,green,blue.
array(
'field' => 'menus_department[]',
'label' => lang('groups_type_department'),
'rules' => array(
'trim',
'required',
'in_list['.implode(array_keys($this->data['groups']['department']),',').']'
)
),