Converting nested array in JSON format with php shows numeric index and not array name

I'm trying to convert a multidimensional php array into JSON format. The problem is that JSON add the "numeric key" of each value of the array.

For constructing the JSON, first I make a query, and then, for append another array which have the business hours of each place, I do another query for each result.

This is the JSON that I get:

{
"sucursales": [
    {
        "sucursal": {
            "id_sucursal": "104",
            "id_user": "2",
            "nombre_sucursal": "wena ql 2",
            "region": "0",
            "0": {
                "dia": "1",
                "hora_inicio": "600",
                "hora_fin": "600"
            },
            "1": {
                "dia": "2",
                "hora_inicio": "600",
                "hora_fin": "600"
            },
            "2": {
                "dia": "3",
                "hora_inicio": "600",
                "hora_fin": "600"
            },
            "3": {
                "dia": "4",
                "hora_inicio": "600",
                "hora_fin": "600"
            }
        }
    }
    .....
]
}

This is the JSON that I want:

{
"sucursales": [
    {
        "sucursal": {
            "id_sucursal": "104",
            "id_user": "2",
            "nombre_sucursal": "wena ql 2",
            "region": "0",
            "horario": {
                "dia": "1",
                "hora_inicio": "600",
                "hora_fin": "600"
            },
            "horario": {
                "dia": "2",
                "hora_inicio": "600",
                "hora_fin": "600"
            },
            "horario": {
                "dia": "3",
                "hora_inicio": "600",
                "hora_fin": "600"
            },
            "horario": {
                "dia": "4",
                "hora_inicio": "600",
                "hora_fin": "600"
            }
        }
    }
    .....
]
}

This is the Code that I use for appending one array to another one. Note that $result is the result of a mysql query.

/* create one master array of the records */
$sucursales = array();
if(mysql_num_rows($result)) {
    while($sucursal = mysql_fetch_assoc($result)) {

        $query_horarios = "SELECT dia,hora_inicio,hora_fin FROM horario_funcionamiento WHERE id_sucursal=".$sucursal['id_sucursal'].";";
        $resultado_horarios = mysql_query($query_horarios,$link) or die('Error en la consulta SQL'); //.$query);

        while($horario = mysql_fetch_assoc($resultado_horarios)){
            $sucursal[]= $horario;
        }
        $sucursales[] = array('sucursal'=>$sucursal);
    }
}

And the code that I use for converting the array to JSON format:

header('Content-type: application/json');
echo json_encode(array('sucursales'=>$sucursales));