PHP函数中变量内部的变量

我有以下函数。

问题是$lang变量将根据$this->setts['site_lang'];而变化。实际问题如下：

$condition_details['description_$lang']

（不起作用）。如何将其显示

$condition_details['description_us']

或

$condition_details['description_fr']

取决于$lang设置？

这里是完整的功能：

function shipped_drop_down($box_name = 'item_shipped_via', $selected = null, $form_refresh = null)
{
    (string) $display_output = null;
    $lang = $this->setts['site_lang'];
    $sql_select_conditions = $this->query("SELECT id, description_".$lang." FROM " . DB_PREFIX . "advanced_shipping_carriers ORDER BY theorder,description_".$lang." ASC");
    $display_output = '<select name="' . $box_name . '" ' . (($form_refresh) ? 'onChange = "submit_form(' . $form_refresh . ', '''')"' : '') . ' style="font-size:10px; width: 120px;"> ';
    while ($condition_details = $this->fetch_array($sql_select_conditions))
    {
        $display_output .= '<option value="' . $condition_details['id'] . '" ' . (($condition_details['id'] == $selected) ? 'selected' : '') . '>' . $condition_details['description_$lang'] . '</option> ';
    }
    $display_output .= '</select> ';
    return $display_output;
}