Only extend classes which will be children of that database object, for instance an SQL-Writer ( Part of an ActiveRecord, ORM Object ) would be a logical extension of a DAL object ( Though the argument could be made that the DAL object should just be fed into the SQL Writer but I digress ).

In this instance the issue seems to be that the object isn't being created in your getInstance() method. Also, I don't see the static instance property in your class.

Hello-world, extending the database class would bear no fruit in this case because the __construct method creates the database connection and it is private ( I'm under the assumption that he would like to keep the current structure )

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