Simple PHP Cookie

I am experimenting with cookies and i am doing this quick example,

<html>
<head>
    <meta charset="UTF-8">
    <title>Cookies</title>
</head>
<body>
    <!-- Start of FORM -->
    <form method="post" action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]); ?>"> 
        Username: <input type="text" name="username"><br>
        <input type="submit" name="submit" value="Submit">
    </form>
    <!-- End of FORM -->
    <hr>
    <?php
    if (isset($_POST['username'] )) {
        setcookie('username', $_POST['username'], time() + 1000, '/');
        if(isset($_COOKIE['username'])){
            echo "Hello " . $_COOKIE['username'];
            unset($_COOKIE['username']);
        }
    }
    ?>
</body>

It works but i have to click the submit button twice for my message to display, why is that?

Solution

From the PHP Docs..

Cookies will not become visible until the next loading of a page that the cookie should be visible for. To test if a cookie was successfully set, check for the cookie on a next loading page before the cookie expires.

So whilst you clicked the button the second time, the actual load was in effect and you were able to see it(the cookie).