//图片上传处理类(这里略)
//upload.php事例代码
namespace backend\components;
class Upload
{
  public function test(){
    $a = '你好';
    return $a;
  }
}

'components' => [
  'imgload' => [
    'class' => 'backend\components\Upload'
  ],
]

public function actionArticlelist()
{
  $cc = Yii::$app->imgload->test();
  var_dump($cc);exit;
}

'components' => [
  'user' => [
    'identityClass' => 'common\models\AgMerch',
    'enableAutoLogin' => true,
  ],
  //自定义图片上传类
  'imgload' => [
    'class' => 'agent\components\Upload'
  ],
]

namespace agent\components;
use yii;
use yii\base\Object;
use yii\web\UploadedFile;
class Upload extends Object
{
  /**
   * [UploadPhoto description]
   * @param [type] $model   [实例化模型]
   * @param [type] $path    [图片存储路径]
   * @param [type] $originName [图片源名称]
   * @param boolean $isthumb  [是否要缩略图]
   */
  public function UploadPhoto($model,$path,$originName,$isthumb=false){
    $root = $_SERVER['DOCUMENT_ROOT'].'/'.$path;
    //返回一个实例化对象
    $files = UploadedFile::getInstance($model,$originName);
    $folder = date('Ymd')."/";
    $pre = rand(999,9999).time();
    if($files && ($files->type == "image/jpeg" || $files->type == "image/pjpeg" || $files->type == "image/png" || $files->type == "image/x-png" || $files->type == "image/gif"))
    {
      $newName = $pre.'.'.$files->getExtension();
    }else{
      die($files->type);
    }
    if($files->size > 2000000){
      die("上传的文件太大");
    }
    if(!is_dir($root.$folder))
    {
      if(!mkdir($root.$folder, 0777, true)){
        die('创建目录失败...');
      }else{
      //  chmod($root.$folder,0777);
      }
    }
    //echo $root.$folder.$newName;exit;
    if($files->saveAs($root.$folder.$newName))
    {
      if($isthumb){
        $this->thumbphoto($files,$path.$folder.$newName,$path.$folder.'thumb'.$newName);
        return $path.$folder.$newName.'#'.$path.$folder.'thumb'.$newName;
      }else{
        return $path.$folder.$newName;
      }
    }
  }
}

use backend\components\Upload;
public function actionArtadd(){
  $model = new Article();
  if ($model->load(Yii::$app->request->post())) {
    //cover为表中的字段名
    $img = Yii::$app->imgload->UploadPhoto($model,'uploads/article/','cover');
    $model->cover = $img;//存入表中
    if($model->save()){
      Yii::$app->getSession()->setFlash('info', '添加成功！');
      return $this->redirect(['xxx']);
    }else{
      Yii::$app->getSession()->setFlash('info', '添加失败！');
      @unlink($img);
      return $this->redirect(['xxx']);
    }
  }
  //代码略
}

<?= $form->field($model, 'cover', [
  'options'=>['class'=>''],
  'inputOptions' => ['class' => 'form-control'],
])->fileInput()->label(false); ?>