define SaveToSQL interface

class ISaveToSQL {
    function GetField()
    {
        echo "hi... go back to study PHP 3 year!";
    }
}

class cucustom extends ISaveToSQL{
    ....

    function GetField()
    {
        $this->field["name"] = $this->name;
        $this->field["tel"] = $this->tel;
        $this->field["address"] = $this->address;

        return $this->field;
    }
}

class CuModelSave extends model
{
    function insertSaveToSQL(/*ISaveToSQL*/ $ISaveToSQL)
    {
        $this->db->insert($this->table_name, $ISaveToSQL->GetField());
    }
}

class custom_sql extends CuModelSave{
    ...
}