如何从数据库中获取逗号分隔值

I have the following mysqli query, and I want to implode the array that's outputted into a (1,2,3,4) type format.

Here's the query and the asoc array code:

$user_categories = mysqli_query($connect, "SELECT sub_cat FROM subscriptions WHERE sub_user_id = '$user_id'");

$category_ids = mysqli_fetch_all($user_categories,MYSQLI_NUM);

print_r($category_ids);

$category_ids = implode(", ",$category_ids);

I then get the following output, and I can't seem to isolate the values...

Array ( 
[0] => Array ( [0] => 5 ) 
[1] => Array ( [0] => 8 ) 
[2] => Array ( [0] => 4 ) 
[3] => Array ( [0] => 2 ) 
)

Apologies if I'm missing something really obvious here. I've been trying to fix this for a while, and due to my lack of PHP experience, I'm not 100% sure what to search for.

I've also tried a simple implode using the results of the $user_categories query, following the instructions of other StackExchange answers I've seen on the topic, but got nothing (code below):

$user_categories = mysqli_query($connect, "SELECT sub_cat FROM subscriptions WHERE sub_user_id = '$user_id'");

$category_ids = implode(", ",$user_categories);

echo $category_ids;