I have just noticed the following while debugging:

Code: $array = array();
Hover mouse over $array shows: $array: array = :undefined = NULL

Code: $array[] = 'first';
Hover mouse over $array shows: $array: array = :undefined = NULL, 0:string = first

There is this spurious entry ':undefined = NULL' which causes problems when I try to iterate all the array entries which are supposed to have non-null keys and non-null values.

Debugger does not change/alter any variables on its own.
I'd recommend you to check what you have in Watch window.

You are correct! My watch window contained $array[$localname] where $localname did not exist. I have removed this watch, and the problem has disappeared.