UploadFile::getInstance($model, $attribute);

UploadFile::getInstances($model, $attribute);

UploadFile::getInstanceByName($name);

UploadFile::getInstancesByName($name);

/**
   * Returns an uploaded file for the given model attribute.
   * The file should be uploaded using [[\yii\widgets\ActiveField::fileInput()]].
   * @param \yii\base\Model $model the data model
   * @param string $attribute the attribute name. The attribute name may contain array indexes.
   * For example, '[1]file' for tabular file uploading; and 'file[1]' for an element in a file array.
   * @return UploadedFile the instance of the uploaded file.
   * Null is returned if no file is uploaded for the specified model attribute.
   * @see getInstanceByName()
   */
  public static function getInstance($model, $attribute)
  {
    $name = Html::getInputName($model, $attribute);
    return static::getInstanceByName($name);
  }

  /**
   * Returns all uploaded files for the given model attribute.
   * @param \yii\base\Model $model the data model
   * @param string $attribute the attribute name. The attribute name may contain array indexes
   * for tabular file uploading, e.g. '[1]file'.
   * @return UploadedFile[] array of UploadedFile objects.
   * Empty array is returned if no available file was found for the given attribute.
   */
  public static function getInstances($model, $attribute)
  {
    $name = Html::getInputName($model, $attribute);
    return static::getInstancesByName($name);
  }

  /**
   * Returns an uploaded file according to the given file input name.
   * The name can be a plain string or a string like an array element (e.g. 'Post[imageFile]', or 'Post[0][imageFile]').
   * @param string $name the name of the file input field.
   * @return UploadedFile the instance of the uploaded file.
   * Null is returned if no file is uploaded for the specified name.
   */
  public static function getInstanceByName($name)
  {
    $files = self::loadFiles();
    return isset($files[$name]) "{$name}[") === 0) {
        $results[] = $file;
      }
    }
    return $results;
  }

/**
   * Creates UploadedFile instances from $_FILE.
   * @return array the UploadedFile instances
   */
  private static function loadFiles()
  {
    if (self::$_files === null) {
      self::$_files = [];
      if (isset($_FILES) && is_array($_FILES)) {
        foreach ($_FILES as $class => $info) {
          self::loadFilesRecursive($class, $info['name'], $info['tmp_name'], $info['type'], $info['size'], $info['error']);
        }
      }
    }
    return self::$_files;
  }

/**
   * Creates UploadedFile instances from $_FILE recursively.
   * @param string $key key for identifying uploaded file: class name and sub-array indexes
   * @param mixed $names file names provided by PHP
   * @param mixed $tempNames temporary file names provided by PHP
   * @param mixed $types file types provided by PHP
   * @param mixed $sizes file sizes provided by PHP
   * @param mixed $errors uploading issues provided by PHP
   */
  private static function loadFilesRecursive($key, $names, $tempNames, $types, $sizes, $errors)
  {
    if (is_array($names)) {
      foreach ($names as $i => $name) {
        self::loadFilesRecursive($key . '[' . $i . ']', $name, $tempNames[$i], $types[$i], $sizes[$i], $errors[$i]);
      }
    } elseif ($errors !== UPLOAD_ERR_NO_FILE) {
      self::$_files[$key] = new static([
        'name' => $names,
        'tempName' => $tempNames,
        'type' => $types,
        'size' => $sizes,
        'error' => $errors,
      ]);
    }
  }

<form class="form-horizontal form-margin50" action="<"
          method="post" enctype="multipart/form-data" id="form1">
         <input type="hidden" name="_csrf" value="<">
         <input type="file" name="head_pic" id="doc" style="display: none" onchange="setImagePreview()"/>
 </form>

public static function uploadImage($userId = '', $tem = '')
  {
    $returnPath = '';
    $path = 'uploads/headpic/' . $userId;
    if (!file_exists($path)) {
      mkdir($path, 0777);
      chmod($path, 0777);
    }

    $patch = $path . '/' . date("YmdHis") . '_';
    $tmp = UploadedFile::getInstanceByName('head_pic');
    if ($tmp) {
      $patch = $path . '/' . date("YmdHis") . '_';
      $tmp->saveAs($patch . '1.jpg');
      $returnPath .= $patch;
    }

    return $returnPath;
  }

class UploadedFile extends Object
{
  /**
   * @var string the original name of the file being uploaded
   */
  // "Chrysanthemum.jpg"
  public $name;
  /**
   * @var string the path of the uploaded file on the server.
   * Note, this is a temporary file which will be automatically deleted by PHP
   * after the current request is processed.
   */
  // "C:\Windows\Temp\php8CEF.tmp"
  public $tempName;
  /**
   * @var string the MIME-type of the uploaded file (such as "image/gif").
   * Since this MIME type is not checked on the server-side, do not take this value for granted.
   * Instead, use [[\yii\helpers\FileHelper::getMimeType()]] to determine the exact MIME type.
   */
  // "image/jpeg"
  public $type;
  /**
   * @var integer the actual size of the uploaded file in bytes
   */
  // 879394
  public $size;
  /**
   * @var integer an error code describing the status of this file uploading.
   * @see http://www.php.net/manual/en/features.file-upload.errors.php
   */
  // 0
  public $error;

  private static $_files;


  /**
   * String output.
   * This is PHP magic method that returns string representation of an object.
   * The implementation here returns the uploaded file's name.
   * @return string the string representation of the object
   */
  public function __toString()
  {
    return $this->name;
  }

  /**
   * Returns an uploaded file for the given model attribute.
   * The file should be uploaded using [[\yii\widgets\ActiveField::fileInput()]].
   * @param \yii\base\Model $model the data model
   * @param string $attribute the attribute name. The attribute name may contain array indexes.
   * For example, '[1]file' for tabular file uploading; and 'file[1]' for an element in a file array.
   * @return UploadedFile the instance of the uploaded file.
   * Null is returned if no file is uploaded for the specified model attribute.
   * @see getInstanceByName()
   */
  public static function getInstance($model, $attribute)
  {
    $name = Html::getInputName($model, $attribute);
    return static::getInstanceByName($name);
  }

  /**
   * Returns all uploaded files for the given model attribute.
   * @param \yii\base\Model $model the data model
   * @param string $attribute the attribute name. The attribute name may contain array indexes
   * for tabular file uploading, e.g. '[1]file'.
   * @return UploadedFile[] array of UploadedFile objects.
   * Empty array is returned if no available file was found for the given attribute.
   */
  public static function getInstances($model, $attribute)
  {
    $name = Html::getInputName($model, $attribute);
    return static::getInstancesByName($name);
  }

  /**
   * Returns an uploaded file according to the given file input name.
   * The name can be a plain string or a string like an array element (e.g. 'Post[imageFile]', or 'Post[0][imageFile]').
   * @param string $name the name of the file input field.
   * @return null|UploadedFile the instance of the uploaded file.
   * Null is returned if no file is uploaded for the specified name.
   */
  public static function getInstanceByName($name)
  {
    $files = self::loadFiles();
    return isset($files[$name]) "{$name}[") === 0) {
        $results[] = new static($file);
      }
    }
    return $results;
  }

  /**
   * Cleans up the loaded UploadedFile instances.
   * This method is mainly used by test scripts to set up a fixture.
   */
  //清空self::$_files
  public static function reset()
  {
    self::$_files = null;
  }

  /**
   * Saves the uploaded file.
   * Note that this method uses php's move_uploaded_file() method. If the target file `$file`
   * already exists, it will be overwritten.
   * @param string $file the file path used to save the uploaded file
   * @param boolean $deleteTempFile whether to delete the temporary file after saving.
   * If true, you will not be able to save the uploaded file again in the current request.
   * @return boolean true whether the file is saved successfully
   * @see error
   */
  //通过php的move_uploaded_file() 方法保存临时文件为目标文件
  public function saveAs($file, $deleteTempFile = true)
  {
    //$this->error == UPLOAD_ERR_OK UPLOAD_ERR_OK 其值为 0，没有错误发生，文件上传成功。
    if ($this->error == UPLOAD_ERR_OK) {
      if ($deleteTempFile) {
        //将上传的文件移动到新位置
        return move_uploaded_file($this->tempName, $file);
      } elseif (is_uploaded_file($this->tempName)) {//判断文件是否是通过 HTTP POST 上传的
        return copy($this->tempName, $file);//copy — 拷贝文件
      }
    }
    return false;
  }

  /**
   * @return string original file base name
   */
  //获取上传文件原始名称 "name" => "Chrysanthemum.jpg" "Chrysanthemum"
  public function getBaseName()
  {
    // https://github.com/yiisoft/yii2/issues/11012
    $pathInfo = pathinfo('_' . $this->name, PATHINFO_FILENAME);
    return mb_substr($pathInfo, 1, mb_strlen($pathInfo, '8bit'), '8bit');
  }

  /**
   * @return string file extension
   */
  //获取上传文件扩展名称 "name" => "Chrysanthemum.jpg" "jpg"
  public function getExtension()
  {
    return strtolower(pathinfo($this->name, PATHINFO_EXTENSION));
  }

  /**
   * @return boolean whether there is an error with the uploaded file.
   * Check [[error]] for detailed error code information.
   */
  //上传文件是否出现错误
  public function getHasError()
  {
    return $this->error != UPLOAD_ERR_OK;
  }

  /**
   * Creates UploadedFile instances from $_FILE.
   * @return array the UploadedFile instances
   */
  private static function loadFiles()
  {
    if (self::$_files === null) {
      self::$_files = [];
      if (isset($_FILES) && is_array($_FILES)) {
        foreach ($_FILES as $class => $info) {
          self::loadFilesRecursive($class, $info['name'], $info['tmp_name'], $info['type'], $info['size'], $info['error']);
        }
      }
    }
    return self::$_files;
  }

  /**
   * Creates UploadedFile instances from $_FILE recursively.
   * @param string $key key for identifying uploaded file: class name and sub-array indexes
   * @param mixed $names file names provided by PHP
   * @param mixed $tempNames temporary file names provided by PHP
   * @param mixed $types file types provided by PHP
   * @param mixed $sizes file sizes provided by PHP
   * @param mixed $errors uploading issues provided by PHP
   */
  private static function loadFilesRecursive($key, $names, $tempNames, $types, $sizes, $errors)
  {
    if (is_array($names)) {
      foreach ($names as $i => $name) {
        self::loadFilesRecursive($key . '[' . $i . ']', $name, $tempNames[$i], $types[$i], $sizes[$i], $errors[$i]);
      }
    } elseif ((int)$errors !== UPLOAD_ERR_NO_FILE) {
      self::$_files[$key] = [
        'name' => $names,
        'tempName' => $tempNames,
        'type' => $types,
        'size' => $sizes,
        'error' => $errors,
      ];
    }
  }
}