How to implement distribution a/b test?

Question

PHP

Hello.
You need to come up with a logic for distributing users.
There are 3 groups and n users.
Group 1 can be entered by 90 percent
. Groups 2 and 3 by 5 percent.
The number of groups and percentages can change, the main thing is that the total should be 100%.
When entering the site, the user is guaranteed to fall into one of the groups, depending on the percentage.
How can this functionality be implemented?

Answer 1

When a user logs in, randomly generate a number from 1 to 100. And assign a group. if dropped out from 1 to 90 - those are the first group. If from 91 to 95 and from 96 to 100 - then 2 and 3 groups, respectively.
You register the group in the database and cookies.
What is the difficulty?