php - Related Models through Junction Table yii2

i have 3 tables for equipments, a table for users, and a table user_equipment_mapping.

Equipments: 1.Laptop: PK - Id, laptop_model, laptop_series, laptop_date_acq 2.Display: PK - ID, display_model, display_series, display_date_acq 3.Phone : PK - ID, phone_model, phone_series, phone_date_acq

User: ID - PK , username, password

User_Equipment_mapping : ID - PK, user_id - FK(id from tbl user), laptop_id - FK(id from tbl laptop), display_id - FK (id from tbl display), phone_id - FK (id from tbl phone), start_date, end_date.

For me this is a many-many relationship.

I want to get in userequipment/create an array of users through junction table so i can assign the equipments to an user using dropdown in gridview. I need your help getting users in create, i have created a method in clas UserEquipmentMapping and tried to print_r($users) in view but it gets me the follwing error : Using $this when not in object context.

Thank you for any suggestions in advance!

Controller :

<?php

namespace app\controllers;

use Yii;
use app\models\UserEquipmentMapping;
use app\models\UserequipmentmappingSearch;
use yii\web\Controller;
use yii\web\NotFoundHttpException;
use yii\filters\VerbFilter;

/**
 * UserequipmentmappingController implements the CRUD actions for UserEquipmentMapping model.
 */
class UserequipmentmappingController extends Controller
{
    /**
     * {@inheritdoc}
     */
    public function behaviors()
    {
        return [
            'verbs' => [
                'class' => VerbFilter::className(),
                'actions' => [
                    'delete' => ['POST'],
                ],
            ],
        ];
    }

    /**
     * Lists all UserEquipmentMapping models.
     * @return mixed
     */
    public function actionIndex()
    {
        $searchModel = new UserequipmentmappingSearch();
        $dataProvider = $searchModel->search(Yii::$app->request->queryParams);

        return $this->render('index', [
            'searchModel' => $searchModel,
            'dataProvider' => $dataProvider,
        ]);
    }

    /**
     * Displays a single UserEquipmentMapping model.
     * @param integer $id
     * @return mixed
     * @throws NotFoundHttpException if the model cannot be found
     */
    public function actionView($id)
    {
        return $this->render('view', [
            'model' => $this->findModel($id),
        ]);
    }

    /**
     * Creates a new UserEquipmentMapping model.
     * If creation is successful, the browser will be redirected to the 'view' page.
     * @return mixed
     */
    public function actionCreate()
    {
        $model = new UserEquipmentMapping();

        if ($model->load(Yii::$app->request->post()) && $model->save()) {
            return $this->redirect(['view', 'id' => $model->id]);
        }

        return $this->render('create', [
            'model' => $model,
        ]);
    }

    /**
     * Updates an existing UserEquipmentMapping model.
     * If update is successful, the browser will be redirected to the 'view' page.
     * @param integer $id
     * @return mixed
     * @throws NotFoundHttpException if the model cannot be found
     */
    public function actionUpdate($id)
    {
        $model = $this->findModel($id);

        if ($model->load(Yii::$app->request->post()) && $model->save()) {
            return $this->redirect(['view', 'id' => $model->id]);
        }

        return $this->render('update', [
            'model' => $model,
        ]);
    }

    /**
     * Deletes an existing UserEquipmentMapping model.
     * If deletion is successful, the browser will be redirected to the 'index' page.
     * @param integer $id
     * @return mixed
     * @throws NotFoundHttpException if the model cannot be found
     */
    public function actionDelete($id)
    {
        $this->findModel($id)->delete();

        return $this->redirect(['index']);
    }

    /**
     * Finds the UserEquipmentMapping model based on its primary key value.
     * If the model is not found, a 404 HTTP exception will be thrown.
     * @param integer $id
     * @return UserEquipmentMapping the loaded model
     * @throws NotFoundHttpException if the model cannot be found
     */
    protected function findModel($id)
    {
        if (($model = UserEquipmentMapping::findOne($id)) !== null) {
            return $model;
        }

        throw new NotFoundHttpException('The requested page does not exist.');
    }
}
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Model :

<?php

namespace app\models;

use Yii;

/**
 * This is the model class for table "user_equipment_mapping".
 *
 * @property int $id
 * @property int $user_id
 * @property int|null $laptop_id
 * @property int|null $phone_id
 * @property int|null $display_id
 * @property string|null $start_date
 * @property string|null $stop_date
 *
 * @property Display $display
 * @property Laptop $laptop
 * @property Phone $phone
 * @property User $user
 */
class UserEquipmentMapping extends \yii\db\ActiveRecord
{
    /**
     * {@inheritdoc}
     */
    public static function tableName()
    {
        return 'user_equipment_mapping';
    }

    /**
     * {@inheritdoc}
     */
    public function rules()
    {
        return [
            [['user_id'], 'required'],
            [['user_id', 'laptop_id', 'phone_id', 'display_id'], 'integer'],
            [['start_date', 'stop_date'], 'safe'],
            [['display_id'], 'exist', 'skipOnError' => true, 'targetClass' => Display::className(), 'targetAttribute' => ['display_id' => 'id']],
            [['laptop_id'], 'exist', 'skipOnError' => true, 'targetClass' => Laptop::className(), 'targetAttribute' => ['laptop_id' => 'id']],
            [['phone_id'], 'exist', 'skipOnError' => true, 'targetClass' => Phone::className(), 'targetAttribute' => ['phone_id' => 'id']],
            [['user_id'], 'exist', 'skipOnError' => true, 'targetClass' => User::className(), 'targetAttribute' => ['user_id' => 'id']],
        ];
    }

    /**
     * {@inheritdoc}
     */
    public function attributeLabels()
    {
        return [
            'id' => 'ID',
            'user_id' => 'User ID',
            'laptop_id' => 'Laptop ID',
            'phone_id' => 'Phone ID',
            'display_id' => 'Display ID',
            'start_date' => 'Start Date',
            'stop_date' => 'Stop Date',
        ];
    }

    /**
     * Gets query for [[Display]].
     *
     * @return \yii\db\ActiveQuery|DisplayQuery
     */
    public function getDisplay()
    {
        return $this->hasOne(Display::className(), ['id' => 'display_id']);
    }

    /**
     * Gets query for [[Laptop]].
     *
     * @return \yii\db\ActiveQuery|LaptopQuery
     */
    public function getLaptop()
    {
        return $this->hasOne(Laptop::className(), ['id' => 'laptop_id']);
    }

    /**
     * Gets query for [[Phone]].
     *
     * @return \yii\db\ActiveQuery|PhoneQuery
     */
    public function getPhone()
    {
        return $this->hasOne(Phone::className(), ['id' => 'phone_id']);
    }

    /**
     * Gets query for [[User]].
     *
     * @return \yii\db\ActiveQuery|UserQuery
     */
    public function getUser()
    {
        return $this->hasOne(User::className(), ['id' => 'user_id']);
    }
    
    public function getUsername()
    {
        return $this->hasMany(User::className(),['user_id' => 'id'])
                    ->viaTable('user');
    }

    /**
     * {@inheritdoc}
     * @return UserEquipmentMappingQuery the active query used by this AR class.
     */
    public static function find()
    {
        return new UserEquipmentMappingQuery(get_called_class());
    }
}
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Create :

<?php

use yii\helpers\Html;

/* @var $this yii\web\View */
/* @var $model app\models\UserEquipmentMapping */

$this->title = 'Create User Equipment Mapping';
$this->params['breadcrumbs'][] = ['label' => 'User Equipment Mappings', 'url' => ['index']];
$this->params['breadcrumbs'][] = $this->title;
?>


<?php 
use app\models\UserEquipmentMapping;
$users =UserEquipmentMapping::getUsername();
print_r($users);die;
?>

<div class="user-equipment-mapping-create">

    <h1><?= Html::encode($this->title) ?></h1>

    <?= $this->render('_form', [
        'model' => $model,
    ]) ?>

</div>
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