文字列値の前後に引用符がありません:
$sql = "INSERT INTO people (person_id, name, username, password, email, salt)
VALUES ($person_id, '$name', '$username', '$password', '$email', '$salt')";
文字列値の前後に引用符がありません:
$sql = "INSERT INTO people (person_id, name, username, password, email, salt)
VALUES ($person_id, '$name', '$username', '$password', '$email', '$salt')";