Collection::reduce versus doing a foreach

Hi!

I'm having trouble wrapping my head around Collection::reduce. Actually, it feels like I understand how it's supposed to work, but in reality it seems to work differently. I have a collection of entities, from which I'm trying to concat a number of string, based on another array (could probably also be an array, I don't think it would matter much). The array has an ID, the collection has that ID as a foreign ID. I loop through the array, and then in each loop I loop through the collection to find the entities which have a foreign ID matching the ID from the array. Implementing this with Collection::reduce gives a wildly different result from doing it with two foreach() loops. Anyway, here's the code:

public function reducetest() {

    $this->render('test');

    $groupby = ['a' => 1, 'b' => 2];

    $array = [

        array('id' => 'b', 'title' => 'array_tag_1'),

        array('id' => 'a', 'title' => 'array_tag_2'),

        array('id' => 'b', 'title' => 'array_tag_3'),

        array('id' => 'a', 'title' => 'array_tag_4'),

        array('id' => 'a', 'title' => 'array_tag_5')

    ];

    $array_result = array();

    foreach ($groupby as $key => $value) {

        $array_result[$key] = '';

        foreach ($array as $item) {

            if ($key === $item['id']) {

                // debug($array_result[$key] . $item['title'] . ", \n");

                $array_result[$key] .= $item['title'] . ', ';

            }

        }

        $array_result[$key] = trim($array_result[$key], ', ');

    }

    debug($array_result);

    $collection = new Collection([

        array('id' => 'b', 'title' => 'collection_tag_1'),

        array('id' => 'a', 'title' => 'collection_tag_2'),

        array('id' => 'b', 'title' => 'collection_tag_3'),

        array('id' => 'a', 'title' => 'collection_tag_4'),

        array('id' => 'a', 'title' => 'collection_tag_5')]

    );

    $collection_result = array();

    foreach ($groupby as $key => $value) {

        $collection_result[$key] = $collection->reduce(function ($string, $item) use ($key) {

            if ($key === $item['id']) {

                // debug($string . $item['title'] . ", \n");

                return $string . $item['title'] . ', ';

            }

        }, '');

        $collection_result[$key] = trim($collection_result[$key], ', ');

    }

    debug($collection_result);

}

The code with the two foreach() loops produces this result, as expected:

[

    'a' => 'array_tag_2, array_tag_4, array_tag_5',

    'b' => 'array_tag_1, array_tag_3'

]

The code that uses the reduce function gives this result, incomprehensibly to me:

[

'a' => 'collection_tag_4, collection_tag_5',

'b' => ''

]

Who can explain to me what I'm not understanding here?

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