Clean up php notices

As it is written, this library emits a bunch of php notices when php is running at higher error reporting levels.  This update cleans up those warnings.

I made one change that deserves critical review.  I changed the conditional statement starting on line 159 in schema-store.php.  The expressions on lines 161 and 162 were causing php notices. It seems that these lines are not really needed.  All tests passed when I removed them, but I am uncomfortable with the change because I'm not sure I fully grasp the intent of the statement.