But2.php

<?php
// Establish a database connection (Assuming you have already connected to your database)

// Assuming the username is obtained from a form submission
$username = $_POST['username']; // Replace this with your actual way of obtaining the username

// Perform the query to check if the user with the given username has a non-null mean
$query = "SELECT username, mean FROM users WHERE username = '$username' AND mean IS NOT NULL";
$result = mysqli_query($connection, $query); // Assuming you are using mysqli

// Check if the query returned any rows
if (mysqli_num_rows($result) > 0) {
    // User with non-null mean found
    header('Location: a.html'); // Redirect to a.html if mean is not null
} else {
    // User not found or mean is null
    header('Location: b.html'); // Redirect to b.html if mean is null or user not found
}

// Close the database connection
mysqli_close($connection); // Assuming you are using mysqli
?>