How to flatten a collection with items of array in Laravel

I need a result of an array that is flattenened like this:

$result = ["1", "2", "3", "4", "5", "6", "7"];

I have a DB result of this query:

    $articles = DB::table('articles')
        ->where('is_published', 'true')
        ->get()
        ->pluck('category');

When I do this with this result:

    $unflattened_array = $articles->toArray();

    dd($unflattened_array);

The result looks like this:

And when I continue with trying to put the ones in arrays and spread then in just one big array as elements:

    $flattened_array = array_merge(...$unflattened_array);

    dd($unflattened_array);

I am getting this error message in Laravel:

array_merge(): Argument #1 must be of type array, string given

What am I doing wrong?

I want the array look like:

["2","3","4","5","6","8","22","45","46","70","100","102","105","106","107","205"]

>Solution :

Your each array element is JSON array, so you must first decode it:

foreach ($unflattened_array as $items) {
    array_push($flattened_array, ...json_decode($items));
}

$flattened_array = array_unique($flattened_array);

Example