Lightweight PHP API Key Authentication: Store and validate user API keys in files

readme for class_APIKey.php

This set of classes is a response to a request at this URL:
https://www.phpclasses.org/recommend/854-A-random-API-key-generator-to-server-as-access-restriction-to-AP.html

There are 3 classes in the set:

Abstract class APIKey
This is the base class that the other classes extend.  It has one method, generate(),
which creates a unique API key.  It also defines two other methods.  Method validate()
test a given API key for validity and returns the name of the person it belongs to, or
throws APIKeyException.  Method storeKey() takes the name of a person, calls generate()
to get a new API key, and stores the name:key combination in a place that validate()
can find.

Class APIKeyValidation
This is the concrete class that provides the two abstract methods called for by APIKey.
When validate() cannot associate an API key with a user name it throws APIKeyException.

Class APIKeyException
Extends Exception and provides a message and an HTTP code.

There are 2 demonstration files in hte set:

Script demo_APIKey.php exercises the classes.
Text File demo_APIKey.txt simulates a database or flat file of valid API keys.

Todo:
Add a method to invalidate an API key.