Form isn't submitting in HTML

I made a login page using HTML and I'm trying to submit the login details to a PHP script using a form. The problem is, that whenever I click the button to submit, it only reloads the page and besides that absolutely nothing.

I am not sure if the data from the form is supposed to show up in the URL, but in my case it doesn't.

Here is the code:

<div class="card-header pb-2 pt-4 text-center">
                    <h2 class="text-uppercase fs-3 fw-bold">login</h2>
                    <p>Login using your username & password</p>
                </div>
                <div class="card-body px-4">
                    <form class="px-3 pb-4" method="POST" action="/includes/login.php">
                        <p class="ms-2 my-2">Username</p>
                        <input class="form-control mb-4" type="text" name="username" placeholder="Type your username here">
                        <p class="ms-2 my-2">Password</p>
                        <input class="form-control mb-4" name="password" type="password" placeholder="Type your password here">
                        <button type="submit" name="submit" class="btn w-100 btn-secondary">Login</button>
                    </form>
                </div>

Here is the beginning of the PHP script:

if($isset($_POST["submit"])) {

And it spits out the following error:

POST /includes/login.php - Uncaught Error: Value of type null is not callable in C:\website\includes\login.php:3

Thanks for help in advance!