Create a json that contains object inside an object in PHP and mysql

What I ultimately want to do is create an api in the format of a json object and pass it along to the front end. And the data is the result of MySQL query.I want to have the structure as this json:

I’ve tried this code

if($num>0){
   
    // cityinfo array
    $city_arr=array();
    $city_arr["records"]=array();

    while ($row = $stmt->fetch(PDO::FETCH_ASSOC)){
        $row['timeline']['cases']
        [$row['Date']] = [$row[`NewCases`]];
       
        extract($row);
        
        $city_item=array(
            "CityName" => $CityName,
            [$row['Date']] = [$row[`NewCases`]], 
        );
        array_push($city_arr["records"], $city_item);
    }
  

but it returns
f

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