I am pretty new to php and js, I saw the similar posts but they were in different languages like react or nodejs, which I haven’t studied yet and thus I don’t understand.

How and why am I getting redirected since I wrote in my php page to send back a json?

How can I get a json in order to extract the location I decided?

Javascript:

const register_endpoint ="../apis/user/register.php";

function isFormCorretto(event){
    event.preventDefault();

//the three functions check if the fields are correct before sending the form, they work as intended
    if(isNomeCorretto() && isPassswordCorretta() && isRipetiCorretta()){
        //send form from here to api for Login;

        const form = new FormData();
        form.append('nome_utente', nome_utente.value);
        form.append('password', password.value);
        form.append('ripeti_password', ripeti_password.value);

        fetch(register_endpoint, {
            method:'post',
            body: form
        }).then(onResponseRegister);
    }
}

function onResponseRegister(response){
    console.log("response di registrazione ricevuta:");
    console.log(response);

    return response.json().then(onJsonRegister);
//stuck ^here^ showing the aforementioned error
}

function onJsonRegister(json){
    console.log("json di registrazione ricevuto:");
    console.log(json);

    if(json.ok){
        console.log(document.location.href);
        window.location.replace("../"+json.new_location+"/");
    }
}

And PHP:

<?php
require "../../session/session.php";

header('Content-Type: application/json; charset=utf-8');

if(!$idUtente=isLogged()){
    header("Location: ../../index/");
    exit;
}

if(!empty($_POST["nome_utente"]) && !empty($_POST["password"]) && !empty($_POST["ripeti_password"])){

    $dbconn=mysqli_connect($dbsettings["host"], $dbsettings["username"], $dbsettings["password"], $dbsettings["dbname"]) /*or die(mysqli_error($dbconn))*/;
    if(mysqli_connect_errno()){
        echo json_encode(array(
            'ok'=>false,
            'error'=>'database_connection'
        ));
        exit();
    }

    if(!preg_match("/^[a-zA-Z0-9_]{3,15}$/", $_POST["nome_utente"])){
        echo json_encode(array(
            'ok'=>false,
            'error'=>'invalid_username'
        ));
        mysqli_close($dbconn);
        exit;
    }
    else {
        $nomeUtente = mysqli_real_escape_string($dbconn, $_POST["nome_utente"]);
        $query = "SELECT nome_utente FROM utenti WHERE nome_utente = '$nomeUtente'";
        $risultato = mysqli_query($dbconn, $query);
        if(mysqli_num_rows($risultato) != 0){
            echo json_encode(array(
                'ok'=>false,
                'error'=>'dupe_username'
            ));
            mysqli_close($dbconn);
            exit;
        }

        if(strlen($_POST["password"])>15 || strlen($_POST["password"])<10){
            echo json_encode(array(
                'ok'=>false,
                'error'=>'invalid_password'
            ));
            mysqli_close($dbconn);
            exit;
        }
    
        if(strcmp($_POST["password"], $_POST["ripeti_password"])!=0){
            echo json_encode(array(
                'ok'=>false,
                'error'=>'not_matching_passwords'
            ));
            mysqli_close($dbconn);
            exit;
        }
        mysqli_free_result($risultato);

    
        $nomeUtente = mysqli_real_escape_string($dbconn, $_POST["nome_utente"]);
        $password = mysqli_real_escape_string($dbconn, $_POST["password"]);
        $password = password_hash($password, PASSWORD_DEFAULT);
    
        $query = "INSERT INTO Utenti(nome_utente, password) VALUES ('$nomeUtente', '$password')";
        
        if(mysqli_query($dbconn, $query)){
            $_SESSION["nome_utente"] = $_POST["nome_utente"];
            $_SESSION["id_utente"] = mysqli_insert_id($dbconn);
    
            echo json_encode(array(
                'ok'=>true,
                'new_location'=>'private'
            ));
            mysqli_close($dbconn);
            exit;
        } else {
            echo json_encode(array(
                'ok'=>false,
                'error'=>'database_connection'
            ));
            mysqli_close($dbconn);
            exit;
        }
    }

} else {
    echo json_encode(array(
        'ok'=>false,
        'error'=>'missing_data'
    ));
    exit;
}
?>

what I get is the private area of the user (the whole PHP/HTML) instead of the expected json [‘ok’=>true,’new_location’=>’private’]