试图将mysql转换为mysqli

Question

我试图将MySql代码转换为MySqli。我的MySql工作得很好，但MySqli代码不起作用（它没有给出结果）。我不知道自己做错了什么，我是＃No;＆＃34;程序员。请你们纠正我的错误。

这是我的MySql代码：

    <?php
session_start();
include 'dbh.php';

$name = $_POST['name'];
$desc = $_POST['desc'];
$placerId = $_SESSION['id'];
$null = "0";

if (isset($_SESSION['uid'])) {
    $sql = "INSERT INTO adds (name, descr, placerId, imgURL)
            VALUES ('$name', '$desc', '$placerId', '$null')";
            $result = mysqli_query($conn, $sql);

    $str = $_POST['keywords'];
    $arr =  explode(" ", $str);

    $sql = "SELECT * FROM adds WHERE placerId='$placerId' ORDER BY id DESC LIMIT 1";
    $result = mysqli_query($conn, $sql);
    $row = mysqli_fetch_assoc($result);

    $addId = $row['id'];

    //print all the value which are in the array
    foreach($arr as $keyword){

        $sql = "INSERT INTO keywords (placerId, productId, keyword)
                VALUES ('$placerId', '$addId', '$keyword')";
                $result = mysqli_query($conn, $sql);

    }

这是我的MySqli代码：

    $name = $_POST['name'];
$desc = $_POST['desc'];
$placerId = $_SESSION['id'];
$null = "0";

if (isset($_SESSION['uid'])) {

    if ($stmt = $mysqli->prepare("INSERT INTO adds (name, descr, placerId, imgURL) VALUES (?, ?, ?, ?)")) {

        // Bind the variables to the parameter as strings. 
        $stmt->bind_param("ssss", $name, $desc, $placerId, $null);

        // Execute the statement.
        $stmt->execute();

        // Close the prepared statement.
        $stmt->close();

    }

    $str = $_POST['keywords'];
    $arr =  explode(" ", $str);

    if ($stmt = $mysqli->prepare("SELECT id FROM adds WHERE placerId=? ORDER BY id DESC LIMIT 1")) {

        // Bind a variable to the parameter as a string. 
        $stmt->bind_param("s", $placerId);

        // Execute the statement.
        $stmt->execute();

        // Get the variables from the query.
        $stmt->bind_result($addId);

        // Fetch the data.
        $stmt->fetch();

        // Close the prepared statement.
        $stmt->close();

        foreach($arr as $keyword){

            if ($stmt = $mysqli->prepare("INSERT INTO keywords (placerId, productId, keyword) VALUES (?, ?, ?)")) {

                // Bind the variables to the parameter as strings. 
                $stmt->bind_param("sss", $placerId, $addId, $keyword);

                // Execute the statement.
                $stmt->execute();

                // Close the prepared statement.
                $stmt->close();

            }
        }

    }

（连接数据库的$ conn变量）代码：

    $conn = mysqli_connect("localhost", "root", "", "stp2");



if (!$conn) {

    die("Connection failed: ".mysqli_connect_error);

}