PHP Shorthand Addition Operator - Undefined Offset

Question

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.

Answer 1

You have to understand that PHP notices are a tool. They exist so you have additional help when writing code and you are able to detect potential bugs easily. Uninitialized variables are the typical example. Many developers ask: if it's not mandatory to initialize variables, why is PHP complaining? Because it's trying to help:

$item_count = 0;
while( do_some_stuff() ){
     $iten_count++; // Notice: Undefined variable: iten_count
}
echo $item_count . ' items found';

Oops, I mistyped the variable name.

$res = mysql_query('SELECT * FROM foo WHERE foo_id=' . (int)$_GET['foo_id']);
// Notice: Undefined index: foo_id

Oops, I haven't provided a default value.

Yours is just another example of the same situation. If you're incrementing the wrong array element, you'd like to know.

Answer 2

Using += (or any of the other augmented assignment operators) assumes that a value already exists for that key. Since this is not the case the first time the ID is encountered, a notice is emitted and 0 is assumed.