在非对象上调用成员函数query();使用全球

我有问题代码:

function get_installed_languages()
{
    global $sql;
    $languages = Array();
    $sql->query("SELECT short_name FROM languages ORDER BY short_name");
    if($sql->getNumRows()==1)
    {
        $languages[0] = "default"; 
        return $languages;
    }
    else
    {
        for ($i=0; $i<$sql->getNumRows(); $i++)
        {
            $get = $sql->getRow($i);
            if(trim($get['short_name'])=='')
                $languages[$i]='default';
            else
                $languages[$i] = strtolower($get['short_name']);
        }
        return $languages;
    }
}

总是返回:

Fatal  error: Call to a member function query() on a non-object

问题行是:

(742): $sql->query("SELECT short_name FROM languages ORDER BY short_name");

完整文件