Error in Ajax with query Mysqli

The problem was in the php part, because of using codeigniter I can not use:

while ($fila = $query->fetch_assoc())
{
    $result += $fila['cash'];
}
echo $result;

I have to use:

foreach ( $query->result_array() as $row)
{
   $result += $row['cash'];

}

echo $result;

With that change works perfectly, thanks for your answers

You execute the query but you are not recovering the results it brings.

It would also add a date-to-date validation, you should escape the data or use prepared statements to avoid SQL injection.

function showVentas()
{
    $fecha = $_POST['fecha'];

    /* Validamos la fecha */
    // dd/mm/aaaa --> 11/01/2015
    $expresion_regular_fecha='/^([0][1-9]|[12][0-9]|3[01])(\/|-)([0][1-9]|[1][0-2])(\d{4})$/'; 
    if (!preg_match($expresion_regular_fecha, $fecha))
    {
        return 'La fecha no es válida';
    }


    $fecha2 = explode("/", $fecha);
    $inicio = $fecha2[2] . "-" . $fecha2[1] . "-" . $fecha2[0] . " 00:00:00";
    $fin    = $fecha2[2] . "-" . $fecha2[1] . "-" . $fecha2[0] . " 23:59:59";

    $statement = "SELECT cash as total FROM pr_gadgets_information WHERE created <= '$fin' && created >= '$inicio'";

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

    $result = 0;

    /* despues de realizar la consulta necesitas traer los resultados */
    while ($fila = $query->fetch_assoc())
    {
        $result += $fila['cash'];
    }

    /*
    foreach ($query as $row)
    {
        $result = $result + $row['cash'];
    }
    */

    echo $result;


}

I also think that if you just want to get the sum of the records you could do it with the SUM function () of SQL directly.