Siverius

//я бы на вашем месте попробовал вот так для начала
        require('connect.php');
        if ($conn->connect_error) {
            die("Connection failed: " . $conn->connect_error);
        }
        if (isset($_POST['username']) && isset($_POST['password'])) {
            $username = $_POST['username'];
            $email = $_POST['email'];
            $password = $_POST['password'];

            $countUsersQuery = 'SELECT count(id) as count_users FROM users WHERE username=?';
            $query = $conn->prepare($countUsersQuery);
            $query->execute([$username]);
            $countUsersResult = $query->fetch();
            if ((int)$countUsersResult['count_users'] === 0) {
                $insertUserQuery = "
                    INSERT INTO users (`username`, `password`, `email`)
                    VALUES ('$username', '$password', '$email')";
            } else {
                echo "<script>alert('Логин уже занят')</script>";
            }

            if ($conn->query($insertUserQuery) === true) {
                header("Location: /");
            } else {
                echo "Error: " . $insertUserQuery . "<br>" . $conn->error;
            }

            $conn->close();
        }

$query = 'SELECT * FROM bonusesstat WHERE dateo BETWEEN ? AND ?';
$execParams[] = $firstDate;
$execParams[] = $lastDate;
R::exec($query, $execParam);

SELECT * FROM table
WHERE id IN (SELECT id FROM table where status = 1 limit 50) 
for update skip locked