framework/grid/Column.php
protected function renderDataCellContent($model, $key, $index)
    {
        if ($this->content !== null) {
            return call_user_func($this->content, $model, $key, $index, $this);
        }

        return $this->grid->emptyCell;
    }
protected function renderDataCellContent($model, $key, $index)
    {
        if ($this->content !== null) {
            return call_user_func($this->content, $model, $key, $index, $this);
        }

        return $this->grid->emptyCell;
    }
Renders the data cell content.
@param mixed $model the data model
@param mixed $key the key associated with the data model
@param int $index the zero-based index of the data model among the models array returned by [[GridView::dataProvider]].
@return string the rendering result