Souke/frontend/models/ProducerCodeForm.php

<?php

namespace frontend\models;

use Yii;
use yii\base\Model;

/**
 * Producer Code form
 */
class ProducerCodeForm extends Model {

    public $id_producer ;
    public $code;

    /**
     * @inheritdoc
     */
    public function rules() {
        return [
            ['id_producer','required','message' => 'Champs obligatoire'],
            ['id_producer', 'integer'],
            ['id_producer', function($attribute, $params) {
                    if ($this->id_producer) {
                        $producer = Etablissement::findOne($this->id_producer);
                        if (!$producer) {
                            $this->addError($attribute, 'Ce producteur n\'existe pas.');
                        }
                    }
                }],
            ['code', 'required', 'message' => 'Champs obligatoire', 'when' => function($model) {
                    $producer = Etablissement::findOne($this->id_producer);
                    if ($producer) {
                        return strlen($producer->code);
                    } else {
                        return false;
                    }
                }],
            ['code', function($attribute, $params) {
                    $code = $this->$attribute;
                    $producer = Etablissement::findOne($this->id_producer);

                    if ($producer && strtolower(trim($code)) != strtolower(trim($producer->code))) {
                        $this->addError($attribute, 'Code incorrect');
                    }
                }],
        ];
    }

    public function attributeLabels() {
        return [
            'id_producer' => 'Producteur',
            'code' => 'Code de l\'établissement',
        ];
    }

}