Simplest Hash: Generate and verify unique hash values

This class can be used to generate and verify unique hash values.

It can be used to prevent that a form is submitted twice or a link be followed more than once.

It generates a random hash value and store it in a MySQL database table. It assures that the hash is unique among all stored hash values.

It can make the hash value be valid for a limited period of time and not be used more than a given limit of times.