Array sum of sales per days in php from mysql

I'm trying to perform a Mysql query to calculate the total sales made in a day and save it in an Array that contains all the days of the month and then load it into chartist.js and create a graph.

    for ($i = 1; $i <= 31; $i++ )
    {
        $año    = date('Y');
        $mes    = date('m');
        $dia    = $i;

        $fechaInicio    = $año . '-' . $mes . '-' . $dia . ' 00:00:00';
        $fechaFin       = $año . '-' . $mes . '-' . $dia . ' 23:59:59';

        $statement = "SELECT SUM(pr_gadgets_information.cash) as total FROM pr_gadgets_information WHERE pr_gadgets_information.created >= '$fechaInicio' && pr_gadgets_information.created <= '$fechaFin'";

        $query = $this->db->query($statement);

        while ($salida = mysqli_fetch_object($query))
        {
            $salida[] = $salida;
        } 
        return $salida;
     }

But I receive the following error: Message: mysqli_fetch_object () expects parameter 1 to be mysqli_result, object given

Any suggestions of where the failure comes from? I am using Codeigniter

I just made the following change:

      for ($i = 1; $i <= 31; $i++ )
        {
            $año    = date('Y');
            $mes    = date('m');
            $dia    = $i;

            $fechaInicio    = $año . '-' . $mes . '-' . $dia . ' 00:00:00';
            $fechaFin       = $año . '-' . $mes . '-' . $dia . ' 23:59:59';

            $statement = "SELECT SUM(pr_gadgets_information.cash) as total FROM pr_gadgets_information WHERE pr_gadgets_information.created >= '$fechaInicio' && pr_gadgets_information.created <= '$fechaFin'";

            $query = $this->db->query($statement);

            foreach ($query->result_array() as $row)
            {
                if($row['total'] == null)
                {
                    $resultado[] = 0; 
                }else {
                    $resultado[] = $row['total'];
                }
            }

        }
        //var_dump($resultado);
        return $resultado;

Performing var_dump ($ row); I get the answers I'm looking for and I also manage the columns that are null, changing them to 0, but when I try to show the data with the return $ result I get the following error: Message: Call to a member function function () on array