Edit the contents of another page with php - php

Question

So I have a simple database for logging in to my website, but i am having trouble with displaying whether or not a user has logged in.
session_start();
$username = $_POST['Username'];
$salt = substr($username, 0, 2);
$password = crypt($_POST['Password'], $salt);
$dbh = new PDO("mysql:host=$dbhost;dbname=$dbname", $dbuser, $dbpass);
$query = $dbh->prepare("SELECT * FROM `7Ducklings` WHERE Username = ? AND Password = ?");
$array = array($username, $password);
$query->execute($array);
$numrows = $query->fetchColumn();
if($numrows == 1)
{
$_SESSION['Username'] = $username;
}else{
}
$dbh = null;
And i want this, if the user is logged in to replace the contents of this div tag:
<div id="duckdiv">
<form id="UserPass" method="POST" action="Check.php">
Username:<input type="text" placeholder="Username" name="Username">Password:<input type="password" placeholder="Password" name="Password">
<img src="ducklogin.png">
</form>
</div>
With this:
<p>"Welcome back:" $_SESSION['Username']</p>
How is this possible?

Answer 1

You have to start the session before using the $_SESSION superglobal array and before rendering any content.
session_start();
if($numrows == 1)
{
$_SESSION['Username'] = $username;
}
HTML view:
<?php if (isset($_SESSION['Username'])) : ?>
// render the welcome message
<?php else : ?>
// render the form
<?php endif ?>