get_where for PDO

I want to change get_where to mysql or PDO, but I do not know! Help ...!

public function get_where($table = '', $where = NULL, $limit = NULL, $offset = NULL)
{
    if ($table !== '')
    {
        $this->from($table);
    }

    if ($where !== NULL)
    {
        $this->where($where);
    }

    if ( ! empty($limit))
    {
        $this->limit($limit, $offset);
    }

    $result = $this->query($this->_compile_select());
    $this->_reset_select();
    return $result;
}

I need help to change:

$this->db->get_where('settings' , array('type' => 'site_name'))->row()->description;

for PDO.

I've changed the following information in the database

Table: settings;

type = > option_name;

site_name = > title

description = > option_value