何时以及如何使用PHP(PDO)使用多个MySQL查询

<?php
// 连接到 MySQL 数据库
$dsn = "mysql:host=localhost;dbname=mydb";
$username = "username";
$password = "password";

try {
    $conn = new PDO($dsn, $username, $password);
    $conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

    // 第一个查询
    $stmt = $conn->prepare("SELECT * FROM table1");
    $stmt->execute();
    $result1 = $stmt->fetchAll(PDO::FETCH_ASSOC);

    // 第二个查询
    $stmt = $conn->prepare("SELECT * FROM table2");
    $stmt->execute();
    $result2 = $stmt->fetchAll(PDO::FETCH_ASSOC);

    // 第三个查询
    $stmt = $conn->prepare("SELECT * FROM table3");
    $stmt->execute();
    $result3 = $stmt->fetchAll(PDO::FETCH_ASSOC);

    // 处理查询结果
    // ...

} catch (PDOException $e) {
    echo "Error: " . $e->getMessage();
}

$conn = null;
?>