Help with header function

I’m building a simple login form that references itself. I’m trying to redirect after a successful login, but my header function isn’t working.

The username password query is working (verified by the echo statements) but I can’t get the header function to work. Any ideas as to what is wrong?

require_once 'scripts/app_config.php';
require_once 'scripts/database_connection.php';


<!doctype html>


	<!-- CSS -->	
	<link rel="stylesheet" href="css/reset.css">
	<link rel="stylesheet" href="css/animate.css">
	<link rel="stylesheet" href="css/login.css">
			$username = $_POST['username'];
			$password = $_POST['password'];
			//Check if the username or password boxes were not filled in
			if(!$username || !$password){
				//if not display an error message
				echo "<center>You need to fill in a <b>Username</b> and a <b>Password</b>!</center>";
				// find user by username and password - working
				$userQuery = 'SELECT * FROM users
                                   WHERE user_name ='.'"'. $username.'" AND password='.'"'. $password.'"' ;
				$users = mysql_query($userQuery);

        		$user = mysql_fetch_array($users);
        		// echo for testing - working - all values are printing
        		echo 'username:'.$user['user_id'].'<br />';
        		echo 'password:'.$user['password'].'<br />';
				echo "<center>You have successfully logged in!</center>";				
				//set the login session storing there id - we use this to see if they are logged in or not
				$_SESSION['user_id'] = $user['user_id'];
				echo "session user_id: ".$_SESSION['user_id'].'<br />';

		<div id="container">
		<form action="" method="POST">
			<label for="username">Username:</label>
			<input type="name" name="username" id="username">
			<label for="password">Password:</label>
			<p><a href="#">Forgot your password?</a>
			<input type="password" name="password" id="password">
			<div id="lower">
				<input type="checkbox"><label class="check" for="checkbox">Keep me logged in</label>
				<input type="submit" value="Login">
			</div> <!-- close lower -->
	<!-- End Page Content -->


The main problem is your echoing out content before the header function has a chance to run, for this function to work correctly nothing can be echoed out before it runs so all your HTML code before your PHP code and echo statements will need to be moved so it loads after the PHP code.