How to allow the 2nd user to take over when the 1st user is logged in?

I am working on a login system where only one user can login and whenever other users try to login, it should give them the warning pop up message saying another user is already logged in - Do you want to take over from them?

• If userA is already logged in and userB tries to login then it should show on the popup as userA is already logged in, do you want to take over?.
• Now for userA behind the scene, we will take away write access and show them another popup message that your write access is revoked.

Second point I can handle it later on but as of now I am focus on doing the point one. Below is my code in which I am trying to achieve the first point at LINE A -

if (isset($_POST['user_name'], $_POST['user_pass']) && $_POST['user_login'] == 1) {
    //Assigning posted values to variables.
    $username = $_POST['user_name'];
    $password = $_POST['user_pass'];

    //Checking the values are existing in the database or not
    $stmt = $connect->prepare("SELECT user_pass FROM userspanel WHERE user_name=?");
    $stmt->bind_param('s', $username);
    $stmt->execute();
    $result = $stmt->get_result();
    $user_from_db = $result->fetch_object();

    if ($user_from_db && password_verify($password, $user_from_db->user_pass)) {

        $_SESSION['user_name'] = $username;

        $sql = "SELECT * FROM trace_users where open='true'";
        $result1 = mysqli_query($connect, $sql);
        if($result1 = mysqli_query($connect, $sql)){
            if(mysqli_num_rows($result1) > 0){
                while($row = mysqli_fetch_array($result1)){
                    if($row['open'] == "true") {
                        if(!isset($_SESSION['pageadmin'])) {
                            $message = "user " . $row['user_name'] . " is logged in. Do you want to take over ?";
                            // LINE A
                            // how to show pop up button which shows $message on it
                            // and add action on ok and cancel button here?
                        }
                        break;
                    }
                }
            } else{
                $_SESSION['pageadmin'] = true;
                $open = "true";
                $read_access = "1";
                $write_access = "1";
                $stmt = $connect->prepare("UPDATE trace_users SET open=?, read_access=?, write_access=? WHERE user_name=?");
                $stmt->bind_param('ssss', $open, $read_access, $write_access, $username);
                $stmt->execute();
            }
        }

    } else {
        echo "Invalid username and password";
    }
}

I want to achieve the following tasks but I am confused on how to achieve it -

• I want to show a pop up message saying "Previous user is logged in. Do you want to take over?" at LINE A.
• Once I click ok button on that pop up message then I want to execute certain code in php let's say print hello world for now and if I click the cancel button on that pop then I don't want to do anything.

I am not able to figure out how to create a pop up box with that message and also add actions on button at LINE A which can do some stuff once I click ok or cancel button.

from How to allow the 2nd user to take over when the 1st user is logged in?