How to make a html button execute a php script ?

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 -

* At `LINE A`, I want to show a pop up message saying `"Previous user is logged in. Do you want to take over?"` with Ok and Cancel buttons.
* Once I click on `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.

In short, I want to achieve two things:

**1.** Create a popup message with Ok and Cancel buttons at Line A.   

**2.** On clicking Ok button, I want to execute php script and on clicking Cancel button I don't want anything to happen.