php如何连接mysql并查询数据

要连接MySQL并查询数据，可以使用PHP中的mysqli扩展或PDO扩展。下面是使用mysqli扩展的示例代码：

<?php
// 连接数据库
$servername = "localhost";
$username = "root";
$password = "password";
$dbname = "mydb";

$conn = new mysqli($servername, $username, $password, $dbname);
if ($conn->connect_error) {
    die("连接失败: " . $conn->connect_error);
}

// 查询数据
$sql = "SELECT * FROM mytable";
$result = $conn->query($sql);

if ($result->num_rows > 0) {
    // 输出数据
    while($row = $result->fetch_assoc()) {
        echo "id: " . $row["id"]. " - Name: " . $row["name"]. " - Age: " . $row["age"]. "<br>";
    }
} else {
    echo "0 结果";
}

// 关闭连接
$conn->close();
?>

使用PDO扩展的示例代码如下：

<?php
// 连接数据库
$servername = "localhost";
$username = "root";
$password = "password";
$dbname = "mydb";

try {
    $conn = new PDO("mysql:host=$servername;dbname=$dbname", $username, $password);
    $conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

    // 查询数据
    $stmt = $conn->prepare("SELECT * FROM mytable");
    $stmt->execute();

    $result = $stmt->setFetchMode(PDO::FETCH_ASSOC);

    if ($stmt->rowCount() > 0) {
        // 输出数据
        while($row = $stmt->fetch()) {
            echo "id: " . $row["id"]. " - Name: " . $row["name"]. " - Age: " . $row["age"]. "<br>";
        }
    } else {
        echo "0 结果";
    }
} catch(PDOException $e) {
    echo "连接失败: " . $e->getMessage();
}

// 关闭连接
$conn = null;
?>

这些示例代码都是基本的连接和查询数据的操作，你可以根据自己的实际情况进行修改和扩展。