Erreur incompréhensible

	                <form action="connexion.php" method="post">
					<label for="" style="color: #fff;">Identifiant</label>
	                	<div class="form-group">
	                		<input class="form-control" type="text" name="username" placeholder="Identifiant" required> 
	                	</div>
					<label for="" style="color: #fff;">Mot de passe</label>					
	                	<div class="form-group">
	                		<input class="form-control" type="password" name="pass" placeholder="Mot de passe" required>
	                	</div>
	                	<div class="form-group">
	                		<button class="btn btn-primary" type="submit"><i class="fa fa-power-off"></i> Se connecter</button>
	                	</div>
	                </form>

<?php
	$username = $_POST['username'];
	$password = md5($_POST['pass']);

	$db = new PDO('mysql:host=localhost;dbname=conferences', 'root', '');

	if(!empty($username) && !empty($password) ){
		$q = $db->query("SELECT * FROM users WHERE username = $username AND pass = $password");
		$data = $q->fetch(PDO::FETCH_ASSOC);

		var_dump($data);
		exit();
		if(!$data){
			$_SESSION['notification'] = "Merci de bien vouloir vérifier votre identifiant ou votre mot de passe";
			header('Location: auth.php');	
		}else{
			session_start();
			$_SESSION['id'] = $data['id'];
			$_SESSION['username'] = $data['username'];
			header('Location: dashboard.php');
		}
	}else{
		$_SESSION['notification'] = "Merci de bien remplir les champs demandées";
		header('Location: auth.php');
	}
?>