implode displays array content as array array instead of values

$trackedTreadIDs[] = array( 'id' =>$getThreadIds['id']  );
                ^^

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You are assigning an array to an array. Did you want this

$trackedTreadIDs = array(
              'id' =>$getThreadIds['id']
            );

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Or this

$trackedTreadIDs[] = $getThreadIds['id'];

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I am guessing the latter - it is the only one that makes sense to use with implode.

implode() will implode a simple array, but it looks like you've got an array of arrays. Can't tell what $getThreadIds['id'] is from your code, so you might want to show us more info. That may well also be an array ??

not really no, im just trying to get two ids in a single array. than implode it

How about:

$result = implode(',', $getThreadIds['id']);