code: protected static function setActivePlaceholder($model, $attribute, &$options = [])
      {
          if (isset($options['placeholder']) && $options['placeholder'] === true) {
              $attribute = static::getAttributeName($attribute);
              $options['placeholder'] = $model->getAttributeLabel($attribute);
          }
      }
docs: Generate placeholder from model attribute label.
  
  @param Model $model the model object
  @param string $attribute the attribute name or expression. See [[getAttributeName()]] for the format
  about attribute expression.
  @param array $options the tag options in terms of name-value pairs. These will be rendered as
  the attributes of the resulting tag. The values will be HTML-encoded using [[encode()]].
  @since 2.0.14