Using Prepared Statement, how I return the id of the inserted row?

After calling the execute() method on the Prepared Statement, the id of the insert row will be in the insert_id attribute.

$pstm->execute();
$pstm->insert_id;

You need to use:

$stmt = $db->prepare("SQL Query");
$stmt->execute();
$id = $db->lastInsertId();

$newid = mysqli_insert_id($mysqli_db);

$mysqli_db is your mysqli database connection. As far as I know it shouldn't matter on which way ou inserted the row (prepared statement or direct INSERT INTO). mysqli_insert_id() should return the id of the last inserted row using this db connection.

The alternative is to make another query like SELECT LAST_INSERT_ID();.