无法获取存储在MySQL数据库中的信息

<?php
include_once 'db_connect.php';
include_once 'psl-config.php';

$error_msg = "";

if (isset($_POST['username'], $_POST['email'], $_POST['twitch'], $_POST['p'])) {
    // Sanitize and validate the data passed in
    $username = filter_input(INPUT_POST, 'username', FILTER_SANITIZE_STRING);
    $email = filter_input(INPUT_POST, 'email', FILTER_SANITIZE_EMAIL);
    $email = filter_var($email, FILTER_VALIDATE_EMAIL);
    $twitch = filter_input(INPUT_POST, 'twitch', FILTER_SANITIZE_STRING);
    if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
        // Not a valid email
        $error_msg .= '<p class="error">The email address you entered is not valid</p>';
    }
    
    $password = filter_input(INPUT_POST, 'p', FILTER_SANITIZE_STRING);
    if (strlen($password) != 128) {
        // The hashed pwd should be 128 characters long.
        // If it's not, something really odd has happened
        $error_msg .= '<p class="error">Invalid password configuration.</p>';
    }

    // Username validity and password validity have been checked client side.
    // This should should be adequate as nobody gains any advantage from
    // breaking these rules.
    //
    
    $prep_stmt = "SELECT id FROM members WHERE email = ? LIMIT 1";
    $stmt = $mysqli->prepare($prep_stmt);
    
    if ($stmt) {
        $stmt->bind_param('s', $email);
        $stmt->execute();
        $stmt->store_result();
        
        if ($stmt->num_rows == 1) {
            // A user with this email address already exists
            $error_msg .= '<p class="error">A user with this email address already exists.</p>';
        }
    } else {
        $error_msg .= '<p class="error">Database error</p>';
    }
    
    // TODO: 
    // We'll also have to account for the situation where the user doesn't have
    // rights to do registration, by checking what type of user is attempting to
    // perform the operation.

    if (empty($error_msg)) {
        // Create a random salt
        $random_salt = hash('sha512', uniqid(openssl_random_pseudo_bytes(16), TRUE));

        // Create salted password 
        $password = hash('sha512', $password . $random_salt);

        // Insert the new user into the database 
        if ($insert_stmt = $mysqli->prepare("INSERT INTO members (username, email, password, salt, twitch) VALUES (?, ?, ?, ?, ?)")) {
            $insert_stmt->bind_param('sssss', $username, $email, $password, $random_salt, $twitch);
            // Execute the prepared query.
            if (! $insert_stmt->execute()) {
                header('Location: ../eh/error.php?err=Registration failure: INSERT');
                exit();
            }
        }
        header('Location: ./register_success.php');
        exit();
    }
}