PDO if语句验证

What am I doing wrong here? All the data was subitting until I added the last part of the code, the IF(){} statement. I want to make it where if the information was submitted correctly, for it to say THANK YOU!, else, display the error i set.

<form method="POST">
    <input type="text" name="username" placeholder="username"><br />
    <input type="password" name="password" placeholder="password"><br />
    <input type="submit">
</form>

<?php
    if(isset($_POST['username'], $_POST['password'])){
        require 'core/db.php';

        $conn = dbConnect()->prepare("INSERT INTO users (username, password) VALUES (?,?)");

        $conn->bindParam(1, $_POST['username']);
        $conn->bindParam(2, $_POST['password']);
        $conn->execute();

        if($conn === true){
            echo 'Thanks!';
            header('Location: index.php');
            exit;
        } else{
            echo 'something went wrong';
        }
    }
?>

It's not the $conn that you should checking for true against. $conn is instance of PDOStatement. You should instead be checking the return value of $conn->execute().

if($conn->execute() === true){
  header('Location: index.php');
  exit;
} else{
  echo 'something went wrong';
}

As @mjayt pointed you should not output content before header call. You could definitely output using output buffering ob_start()...ob_end_flush(), but I do not assume the echo is really necessary here.

if($conn->execute()){
  header('Location: index.php?success=true');
  exit;
} else{
  echo 'something went wrong';
}

I would just do this, the echo won't work, it will break the header() redirect because you are sending content to the browser. Even if it did, you wouldn't see it because it will have redirected you to the other page. I usually add a flag to the redirect so I can display a success message there.