PHP Shorthand Addition Operator - Undefined Offset

I'm using the PHP shorthand addition operator to tally the number of times a specific id occurs within a multidimensional array:

$source['tally'] = array();

foreach ($items as $item) {
    $source['tally'][$item->getId()] += 1;
}

The first time it hits a new id, it sets its 'tally' value to 1 and then increments it each time it's found thereafter.

The code works perfectly (I get the correct totals), but PHP gives me an "Undefined Offset" notice each time it finds a new id.

I know I can just turn off notices in php.ini, but figured there must be a reason why PHP doesn't approve of my technique.

Is it considered bad practice to create a new key/offset dynamically like this, and is there a better approach I should take instead?

Please Note: To help clarify following initial feedback, I do understand why the notice is being given. My question is whether I should do anything about it or just live with the notice. Apologies if my question didn't make that clear enough.

Solution

If you simply want to defer the diagnostics (warning, notice), you can use an Immediately Invoked Function Expression (IIFE) that initializes return parameters:

$source['tally'] = array();

foreach ($items as $item) {
    (fn(&$_) => $_++)($source['tally'][$item->getId()]);
}

This has the benefit, that you would still get a warning on $source['tally'] if tally was not initialized.

Which is not the case when using the error control operator, that eats all messages:

    @$source['tally'][$item->getId()]++;

However, you should generally initialize your variables, in this case by adding the following code inside the loop (Null Coalesce assignment operation):

if (!isset( $source['tally'][$item->getId()] ))
{
   $source['tally'][$item->getId()] ??= 0;
   $source['tally'][$item->getId()]++;
}

Or perhaps in your case, map the ids and count:

$source['tally'] = array_count_values(
    array_map(fn($item) => $item->getId(), $items)
);