problema na criação de página de registro

<!-- Registration with database-->
<?php

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


?>

<!DOCTYPE html>
<html lang="pt-br">
<head>
	<meta charset="UTF-8">
	<title>Registro de pessoas</title>
	<link rel="stylesheet" href="bower_components/bootstrap/dist/css/bootstrap.css">
	<style>
		.jumbotron {
		    margin-left: auto;
		    margin-right: auto;
		    width: 50%;
		    padding: 0 10%;
		}
	</style>
</head>
<body>

<div class="jumbotron">
	<h1> Registro </h1>

<?php

	if(empty($_POST)){

?>

	<form name="registration" action="registration.php" method="POST">


		  <div class="form-group">
		    <label for="nome">UserName: </label>
		    <input type="text" name="username" class="form-control" id="exampleInputEmail1" placeholder="UserName">
		  </div>

		  <div class="form-group">
		    <label for="senha">Password</label>
		    <input type="password" name="password" class="form-control" id="exampleInputPassword1" placeholder="Password">
		  </div>

		  <div class="form-group">
		    <label for="first_name">Firt name: </label>
		    <input type="text" name="first_name" class="form-control" id="exampleInputPassword1" placeholder="Firt name">
		  </div>

		  <div class="form-group">
		    <label for="surname">Surname: </label>
		    <input type="text" name="surname" class="form-control" id="exampleInputPassword1" placeholder="Surname">
		  </div>

		  <div class="form-group">
		    <label for="address">Address: </label>
		    <input type="text" name="address" class="form-control" id="exampleInputPassword1" placeholder="Address">
		  </div>

		  <div class="form-group">
		    <label for="email">E-mail: </label>
		    <input type="text" name="email" class="form-control" id="exampleInputPassword1" placeholder="E-mail">
		  </div>


		  <button type="submit" class="btn btn-default btn-primary">Submit</button>
	</form>

<?php

	$form = $_POST;
	$username = $form['username'];
	$password = $form['password'];
	$first_name = $form['first_name'];
	$surname = $form['surname'];
	$address = $form['address'];
	$email = $form['email'];

	echo $form;

	$sql = "INSERT INTO usuarios (username, password, first_name, surname, address, email ) VALUES ( :username, :password, :first_name, :surname, :address, :email )";

	$query = $db->prepare( $sql );
	$query->execute( array( ':username'=>$username, ':password'=>$password, ':first_name'=>$first_name, ':surname'=>$surname, ':address'=>$address, ':email'=>$email ) );

	$result = $query->execute( array( ':username'=>$username, ':password'=>$password, ':first_name'=>$first_name, ':surname'=>$surname, ':address'=>$address, ':email'=>$email ) );


	}else{

		if ( $result ){
		  echo "<p>Thank you. You have been registered</p>";
		} else {
		  echo "<p>Sorry, there has been a problem inserting your details. Please contact admin.</p>";
		}

	}

?>
</div>
</body>
</html>