mysql进度条不动

<?php
$servername = "localhost";
$username = "username";
$password = "password";
$dbname = "myDB";

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

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

// 长时间运行的查询
$sql = "SELECT * FROM large_table";
$result = $conn->query($sql);

// 更新进度条
while ($row = $result->fetch_assoc()) {
    // 处理每一行数据
    // ...

    // 更新进度条
    echo "<script>updateProgressBar(" . ($result->num_rows - $result->fetch_row()) / $result->num_rows * 100 . ");</script>";
    ob_flush();
    flush();
}

$conn->close();
?>