PHP shows POST data sent by XMLHttpRequest does not exist?

I am currently trying to create a signup page on XAMPP. The summary of my system is that when user clicks submit button, JS performs xmlhttprequest to a php file via POST that uploads the user's data to database and creates a new account. However, after some debugging, it became clear that the php file seemed to not have received any of the POST data.

my JS:

form.addEventListener("submit", function (ev) {
    ev.preventDefault();
    xhr = new XMLHttpRequest();
    xhr.timeout = 10000;
    xhr.open("POST", "/db/signup.php");
    xhr.setRequestHeader("Content-Type", "multipart/form-data; boundary=---------------------------974767299852498929531610575");
    xhr.addEventListener("timeout", function () {
        errorMsg("Timed out. Try again later.")
    }, false);
    xhr.addEventListener("readystatechange", 
        function () {
            if (xhr.readyState == 4) {
                if (xhr.status == 200 && xhr.responseText !== null) {
                    // if (xhr.responseText.startsWith("ERROR: ")) {             regular code
                    //     loadingIcon.style.opacity = "0";
                    //     errorMsg(xhr.responseText.slice(6));
                    // } else {
                    //     location.href = "/?message=Created%20account!";
                    // }
                    errorMsg(xhr.responseText);   //                             debugging code
                } else {
                    location.href = "/500.html";
                }
            }
        },
    false);
    xhr.send(new FormData(form));
}, false);

my PHP:

try {
  $statement = $pdo->prepare("INSERT INTO users (name, email, password, birthday, imgpath) VALUES (:n, :e, :p, :dob, :imgpath)");
  $statement->execute(
    ["n" => $_POST["name"], "e" => $_POST["email"],"p" => password_hash($_POST["password"], PASSWORD_BCRYPT), "dob" => $_POST["dob"], "imgpath" => $path ?? ""]);
} catch (PDOException $e) {
  if ($e->errorInfo[1] == 1062) {
    $error = "Already an account with email \"" . htmlspecialchars($_POST["email"], ENT_QUOTES | ENT_HTML5) . "\"";
  } else {
    $error = "An error occured. Try again later.";
  }
}

NOTE: These are only parts of my entire code

I hope you can find a solution to my problem. All help is appreciated!

Remove

xhr.setRequestHeader("Content-Type", "multipart/form-data; boundary=---------------------------974767299852498929531610575");

from your Javascript code. You don't need to set this manually, XHR will take care of it for you and ensure it's done correctly.