Yii Framework File Upload

Question

I'm trying to learn how to upload an image file in Yii. I'm using this code

    <?php
use yii\widgets\ActiveForm;
?>

<?php $form = ActiveForm::begin(['options' => ['enctype' => 'multipart/form-data']]) ?>

<?= $form->field($model, 'imageFile')->fileInput() ?>

    <button>Submit</button>

<?php ActiveForm::end() ?>

in ProjectFile/views/site/upload.php file. The problem is in

<?= $form->field($model, 'imageFile')->fileInput() ?>

$model gives me a red underline. I have looked many examples and all of them wrote the like this. What I need to do stop this problem?

Edit: Is inside the controller/SiteController.php

// function for upload
    public function actionUploadImage()
    {
        $model = new UploadImageForm();

        if (Yii::$app->request->isPost) {
            $model->imageFile = UploadedFile::getInstance($model, 'imageFile');
            if ($model->upload()) {
                // file is uploaded successfully
                return;
            }
        }

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

Is inside of models/UploadImageForm.php

<?php


namespace app\models;

use yii\base\Model;
use yii\web\UploadedFile;

class UploadImageForm extends \yii\base\Model
{

    public $imageFile;

    // gives rules of how to upload picture
    public function rules(){
        return [
          [['imageFile'], 'file', 'skipOnEmpty' => false, 'extensions' => 'png, jpg'],
        ];
    }

    // uploads picture
    public function upload(){
        if($this->validate()){
            $this->imageFile->saveAs('uploads/' . $this->imageFile->baseName . '.' . $this->imageFile->extension);
        }
    }

}

Answer 1

In views\upload.php the red underline is shown because the system can't find the $model. While running, program will connect $model to controller. So the red underline is not a problem for php files.