mysql 查询语句占位符

<?php
$servername = "localhost";
$username = "username";
$password = "password";
$dbname = "myDB";

// 创建连接
$conn = new mysqli($servername, $username, $password, $dbname);

// 检查连接
if ($conn->connect_error) {
    die("连接失败: " . $conn->connect_error);
}

// 使用占位符的预处理语句
$stmt = $conn->prepare("SELECT * FROM users WHERE username = ?");
$stmt->bind_param("s", $username);

$username = "john_doe";

$stmt->execute();

$result = $stmt->get_result();

while ($row = $result->fetch_assoc()) {
    echo "ID: " . $row["id"]. " - Username: " . $row["username"]. "<br>";
}

$stmt->close();
$conn->close();
?>

$stmt->bind_param("s", $username); // 确保参数类型为字符串

if ($stmt->execute()) {
    echo "查询成功";
} else {
    echo "查询失败: " . $stmt->error;
}

$stmt = $conn->prepare("SELECT * FROM users WHERE id = ?");
$stmt->bind_param("i", $id); // 确保参数类型为整数