Increase value by 1 on button click

PHP Snippet 1:

<form action="" method="post">
<input type="submit" name="click_button" value="Click..">
</form>
<?php
session_start();

if (isset($_POST['click_button'])) {
    $_SESSION['clicks'] += 1 ;
} else {
    $_SESSION['clicks'] = 0;
}

echo($_SESSION['clicks']);