collection.php

<?php

collection($items)
    // I only care about elements having the id = a or id = b
    ->filter(function ($value, $key) use ($array) {
        return isset($array[$value['id']]); // If the key is in the array, the it should be kept in the collection
    })

    /// Let's now index the collection by the id
    ->groupBy('id')

    // Finally, let's reduce the collection to list of strings per key

    ->reduce(function ($result, $value) {
        return $result . ', ' . $value['title'];
    }, '')