Why PHP can't recognize when the input[type='button'] is clicked on?

I just want to simply echo 'OK!' when the input with type="button" name="calculateBtn" is clicked on, but PHP does not recognize that with the condition if (isset($_POST['calculateBtn'])) so wouldn't do anything, can somebody please tell me why is that?

<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
</head>
<body>
<div class="container">
    <div class="row">
        <form action="" method="post">
            <div>
                <input id="input" type="text">
                <?php
                if (isset($_POST['calculateBtn'])) {
                    echo 'ok';
                }
                ?>
                <input name="calculateBtn" value="change the value" type="button">
            </div>
        </form>
    </div>
</div>
</body>
</html>

Solution

First, you don't have an action defined in the form, second the button type is set as a button instead of submit, it will not submit the form.

<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
</head>
<body>
<div class="container">
    <div class="row">
        <form action="page.php" method="post">
            <div>
                <input id="input" type="text">
                <?php
                if (isset($_POST['calculateBtn'])) {
                    echo 'ok';
                }
                ?>
                <input name="calculateBtn" value="change the value" type="submit">
            </div>
        </form>
    </div>
</div>
</body>
</html>

please set the page.php to the name of your page(file).