Mysqli Prepare is not working inside another prepared statement

Question

I am trying to run a prepared statement inside a prepared statement but got no success. Here is my code:

if ($stmt = $mysqli->prepare("SELECT author_id FROM posts")) {
$stmt->execute();
$stmt->bind_result($author_id);
    while ($stmt->fetch()) {
        if ($stmt2 = $mysqli->prepare("SELECT username FROM users WHERE id=? LIMIT 1")) {
            $stmt2->bind_param("s", $author_id);
            $stmt2->execute();
            $stmt2->bind_result($username);
            $stmt2->fetch();
            $stmt2->close();

            //showing username
            echo $username;
        }
    }
    $stmt->close();
}

I am getting author id from a table and then from author id I'm trying to get author's name from another table.
Can you please tell me any way to do this or any modification in this script can get it done.

Answer 1

Sometimes PHP section of this site makes my eyes aching. Not only question lacks very basic knowledge in every aspect of programming, but also answers promote terrible practices.

1. You have to report errors to make yourself aware of the certain reason. Either usual way, via $conn->error or by setting mysqli in exception mode.
2. After reading the error message, you can google for a solution - store_result()
3. For such a dataset, NO loop needed ever, as you have to use JOIN instead:

And so whole code become

mysqli_report(MYSQLI_REPORT_ERROR | MYSQLI_REPORT_STRICT);

$sql = "SELECT username FROM posts p, users u WHERE u.id=author_id";
$res = $mysqli->query($sql);
while ($row = mysqli_fetch_row()) {
    echo $row[0];
}

Answer 2

Use another mysqli object for inner loop.

$mysqli = new mysqli(host, user, password, dbname);
$mysqli2 = new mysqli(host, user, password, dbname);

if ($stmt = $mysqli->prepare("SELECT author_id FROM posts")) {
$stmt->execute();
$stmt->bind_result($author_id);
while ($stmt->fetch()) {
    if ($stmt2 = $mysqli2->prepare("SELECT username FROM users WHERE id=? LIMIT 1")) {
        $stmt2->bind_param("s", $author_id);
        $stmt2->execute();
        $stmt2->bind_result($username);
        $stmt2->fetch();
        $stmt2->close();

        //showing username
        echo $username;
    }
}
$stmt->close();