Create or reorder an array based on values from array of objects

I have an array of objects, a portion of which looks like this:

[Results] => Array
    (
        [0] => stdClass Object
            (
                [Id] => 3103
                [Tag] => Atticus Ross
            )

        [1] => stdClass Object
            (
                [Id] => 3105
                [Tag] => Atticus Ross
            )

        [2] => stdClass Object
            (
                [Id] => 3106
                [Tag] => Grant Marshall
            )

        [3] => stdClass Object
            (
                [Id] => 3107
                [Tag] => Trent Reznor
            )

        [4] => stdClass Object
            (
                [Id] => 3108
                [Tag] => Atticus Ross
            )

        [5] => stdClass Object
            (
                [Id] => 3109
                [Tag] => Atticus Ross
            )

        [6] => stdClass Object
            (
                [Id] => 3110
                [Tag] => Grant Marshall
            )
        [7] => stdClass Object
            (
                [Id] => 3114
                [Tag] => Trent Reznor
            )    
    )

[Id] is unique.

[Tag] is not unique.

I need to restructure this so I know which Ids correspond to each Tag. Each Tag should only be listed once. Something like:

Array
(
    [Atticus Ross] => Array
        (
            [0] => 3103
            [1] => 3105
            [2] => 3108
            [3] => 3109
        )
    [Grant Marshall] => Array
        (
            [0] => 3106
            [1] => 3110
        )   
    [Trent Reznor] => Array
        (
            [0] => 3107
            [1] => 3114
        )        
)

It doesn’t necessarily need to be structured like that, but hopefully makes it easier to see what I’m looking for.

I’ve tried looping through the original array, and I’m able to get a unique array of Tags, but I’m not sure how to add/merge the Ids for each Tag:

$all_tags = array();

foreach ($results as $result) {
    $Id = $result->Id;
    $Tag = $result->Tag;

    $all_tags[] = $Tag;
    $all_tags_unique = array_values(array_unique($all_tags));
}

I may be way off here. Appreciate any help!

Answer

It should be enough just to loop through the result elements and add them to an element identified by the Tag…

$all_tags = array();

foreach ($results as $result) {
    $all_tags[ $result->Tag ][] = $result->Id;
}

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Answer