Fix for php 8.1.2 / Ubuntu 22.04

[hjtappe]

Make script usable in recent environment: Add robustness to remove PHP warnings and deprecated bahavior.

[thomascube]

The add operator + is not equal to array_merge(): "for keys that exist in both arrays, the elements from the left-hand array will be used" whereas with array_merge, the second argument wins.

What's the problem with the add operator in PHP 8 (haven't tested it)?

[thomascube]

Hello @hjtappe. Thanks a lot for your work! I somehow lost track with the updates of PHP and I'm happy for your contribution. The only thing I'm not sure is the replacement of the + array operator with array_merge() (see inline comment)

[hjtappe]

Hello @thomascube,

1. Not initializing the four arrays led to many PHP warnings about undefined array members, so I initialized the arrays with the expected key-value pairs.
2. Now, using the + operator on the pre-initialized array keeps the original (empty) values.
3. I don't see that not overwriting an existing value would change the program behavior because it is an associative array, which can hold only one value per key anyways. That would be different on plain value-only arrays, which would grow in that case.

So, we could only revert back to the + operator if we check all use places of array keys with a defined() call, which I deem unnecessary for what the program does at these lines. Just replacing the array_merge with the plus operator will now lead to missing data in the export output.