如何用PHP合并两个sql查询

<?php
// 数据库连接信息
$servername = "localhost";
$username = "username";
$password = "password";
$dbname = "database";

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

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

// 第一个SQL查询
$sql1 = "SELECT column1, column2 FROM table1";

// 第二个SQL查询
$sql2 = "SELECT column3, column4 FROM table2";

// 合并两个SQL查询
$sql = $sql1 . " UNION " . $sql2;

// 执行查询
$result = $conn->query($sql);

// 检查查询结果
if ($result->num_rows > 0) {
    // 输出每一行数据
    while($row = $result->fetch_assoc()) {
        echo "column1: " . $row["column1"]. " - column2: " . $row["column2"]. "<br>";
    }
} else {
    echo "0 结果";
}

// 关闭连接
$conn->close();
?>