在使用PDO查詢數據時，可以按照以下步驟進行操作：

1. 連接到數據庫：

$servername = "localhost";
$username = "root";
$password = "";
$dbname = "database_name";

try {
    $conn = new PDO("mysql:host=$servername;dbname=$dbname", $username, $password);
    $conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    echo "Connected successfully";
} catch(PDOException $e) {
    echo "Connection failed: " . $e->getMessage();
}

2. 編寫SQL查詢語句：

$sql = "SELECT * FROM table_name";

3. 準備并執行查詢語句：

$stmt = $conn->prepare($sql);
$stmt->execute();

4. 獲取查詢結果：

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

5. 處理查詢結果：

foreach($result as $row) {
    echo $row['column_name'];
}

完整的示例代碼如下：

$servername = "localhost";
$username = "root";
$password = "";
$dbname = "database_name";

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

    $sql = "SELECT * FROM table_name";
    $stmt = $conn->prepare($sql);
    $stmt->execute();
    $result = $stmt->fetchAll(PDO::FETCH_ASSOC);

    foreach($result as $row) {
        echo $row['column_name'];
    }
} catch(PDOException $e) {
    echo "Connection failed: " . $e->getMessage();
}

注意替換$servername、$username、$password和$dbname為實際的數據庫連接信息。同時，將table_name和column_name替換為實際的表名和列名。