Unknown type 18 sent by the server - example

<?php

class Database {
    private $host, $dbname, $user, $password, $connection = null;

    // Open database connection
    public function connect($host, $dbname, $user, $password) {
        $connectionString = sprintf("mysql:host=%s;dbname=%s;charset=utf8mb4", $host, $dbname);

        try {
            $this->connection = new PDO($connectionString, $user, $password);

            $this->connection->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);
            $this->connection->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

            $this->connection->query("SET sql_mode = ''");
        } catch (PDOException $pe) {
            die("Database error: ".$pe->getMessage());
        }
    }

    public function get_board_threads() {
        global $cfg;

        $query = "
            SELECT posts.id, posts.thread, posts.date, posts.header, posts.text
            FROM posts
            WHERE posts.thread = 0
            GROUP BY posts.id
            ORDER BY posts.last_reply DESC";

        $q = $this->connection->prepare($query);
        $q->execute();
        return $q->fetchAll();
    }

    // Working function but not what I want
    public function get_thread_latests_replies($id) {
        global $cfg;

        $query = "
            SELECT posts.id, posts.thread, posts.date, posts.header, posts.text
            FROM posts
            WHERE posts.thread = :thread
            GROUP BY posts.id
            ORDER BY posts.id ASC LIMIT 3";

        $q = $this->connection->prepare($query);
        $q->execute(array(":thread" => $id));
        return $q->fetchAll();
    }

    // Error is created here
    public function get_thread_latests_replies_error($id) {
        global $cfg;

        $query = "
            (SELECT posts.id, posts.thread, posts.date, posts.header, posts.text
            FROM posts
            WHERE posts.thread = :thread
            GROUP BY posts.id
            ORDER BY posts.id ASC LIMIT 3) ORDER BY id DESC";

        $q = $this->connection->prepare($query);
        $q->execute(array(":thread" => $id));
        return $q->fetchAll();
    }

    // Close the database connection
    public function __destruct() {
        $this->connection = null;
    }
}



/* Some "cosmetic" functions */
function print_thread($thread) {
    $thread_html = "<h3>".$thread[0]["header"]."</h3>";

    $thread_html .= print_post($thread[0], true)."<div class='answers'>";

    for ($i = 1; $i < count($thread); $i++) {
        $thread_html .= print_post($thread[$i]);
    }

    $thread_html .= "
            </div>
        </div>";

    return $thread_html;
}

function print_post($post, $start_post = false) {
    $html = "
        <div style='border: 1px solid #000; margin: 3px;'>
            <div>#".$post["id"]." | ".$post["date"]."</div>
            <div>
                <p>".nl2br($post["text"])."</p>
            </div>";

    if ($start_post == false) {
        $html .= "</div>";
    }

    return $html;
}

/* End of "cosmetic" functions */



echo "<h2>Run this SQL query in your MySQL Server first as root user</h2>

<pre>
CREATE DATABASE IF NOT EXISTS test_db DEFAULT CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci;

CREATE USER 'test_user'@'localhost' IDENTIFIED BY 'safepw123';

GRANT ALL ON `test_db`.* TO 'test_user'@'localhost';

FLUSH PRIVILEGES;

USE test_db;

CREATE TABLE `posts` (
    `id` int(11) NOT NULL,
    `thread` int(11) NOT NULL DEFAULT '0',
    `date` datetime NOT NULL,
    `last_reply` datetime DEFAULT NULL,
    `header` varchar(32) NOT NULL DEFAULT '',
    `text` varchar(8000) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

INSERT INTO `posts` (`id`, `thread`, `date`, `last_reply`, `header`, `text`) VALUES
    (1, 0, '2016-04-17 14:31:28', '2016-04-17 15:52:33', 'Title', 'start post (always top/first)\r\n\r\nThe order should be 6, 7, 8 from top to bottom. So only the latest three messages should be showing'),
    (2, 1, '2016-04-17 14:31:56', NULL, '', '2nd post'),
    (3, 1, '2016-04-17 14:51:31', NULL, '', '3rd post'),
    (4, 1, '2016-04-17 14:55:42', NULL, '', '4th post'),
    (5, 1, '2016-04-17 15:34:01', NULL, '', '5th post'),
    (6, 1, '2016-04-17 15:43:44', NULL, '', '6th posts'),
    (7, 1, '2016-04-17 15:49:12', NULL, '', '7th posts'),
    (8, 1, '2016-04-17 15:52:33', NULL, '', '8th posts');

ALTER TABLE `posts`
    ADD UNIQUE KEY `id` (`id`),
    ADD KEY `thread` (`thread`);

</pre>";

echo "<br><br><h2 style='color:red;'>To create a PHP error add \"?error=1\" in the url</h2>";

echo "<br><br><h2>PHP Output:</h2>";

/* Connect to database */
$db = new Database();
$db->connect("127.0.0.1", "test_db", "test_user", "safepw123");

$threads = $db->get_board_threads();
$threads_html = "";

foreach ($threads as $thread) {
    if (isset($_GET["error"]) AND $_GET["error"] == 1) {
        // Creates error
        $replies = $db->get_thread_latests_replies_error($thread["id"]);
    } else {
        // Doesn't create error
        $replies = $db->get_thread_latests_replies($thread["id"]);
    }

    $threads_html .= print_thread(array_merge(array($thread), $replies));
}

// Print the output
echo "<div style='width: 300px;'>".$threads_html."</div>";

?>