Mysql group by order while rows with same reference but not all

Question

i have simple table

id	order_id	user_id
1	34	2
2	42	2
3	45	3
4	46	3
5	47	2

i want to group order_ids by user_id order. so i will have result like this

1. 34,42
2. 45,46
3. 47

so make group order_ids while user_ids are same. then start grouping again