如何使用PHP获取列中的所有值？

$servername = "localhost";
$username = "username";
$password = "password";
$dbname = "myDB";

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

$sql = "SELECT columnName FROM myTable";
$stmt = $conn->prepare($sql);
$stmt->execute();

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

foreach ($result as $value) {
    echo $value . "<br>";
}

$servername = "localhost";
$username = "username";
$password = "password";
$dbname = "myDB";

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

    $sql = "SELECT columnName FROM myTable";
    $stmt = $conn->prepare($sql);
    $stmt->execute();

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

    foreach ($result as $value) {
        echo $value . "<br>";
    }
} catch(PDOException $e) {
    echo "Connection failed: " . $e->getMessage();
}