为什么数据库不能反映php INSERT所做的更改？

I am calling INSERT and DELETE Statements via PHP to a MySQL Database. The PHP script finishes successfully, but in the database nothing has changed. So I believe it must have to do with transaction handling. But I start and end the transaction. Does anyone has an idea? Thanks a lot.

<?php
require_once '../extra/pdo_connect.php';
$postdata = file_get_contents("php://input");
if (isset($postdata) && !empty($postdata)) {
    $json_obj = json_decode($postdata,true);

    $pdo->beginTransaction();

    $sql  = "delete from user where username = '?'";
    $stmt = $pdo->prepare($sql);
    $rows = $stmt->execute(array($json_obj['username']));
    $echo("Statement executed");

    $pdo->commit();
}
?>

As the "Statement executed" is in the Response, the DML and the transaction should have been sucessful.

Nigels remark was the key. Instead of '?' I just used ?. After taht it worked well.

Thanks a lot for your help!!