About PHP ID similar to stagger a certain number of writes about the problem

Questions about how to stagger a certain number of writes with similar PHP IDs
A seating chart, a student table

Seating chart:
ID Zid UserID

Student table:
UserID Cityid


Now I want to randomly write the student table students to the seating chart, seating chart Zid is a 2 digit seat number

For example, the student table has 20 students, of which 5 cities Cityid the same, 5 Cityid the same student to stagger 5 digits in the seating table Zid so, how can it be achieved?

A PHP for testing purposes to prevent the city Cityid the same student seat.

Thank you, first!
------to solve the idea----------------------
Stagger 5 bits is the interval between 5 and above, or the interval 4 can be?
If it is the former, then 5 students of the same Cityid will not be able to appear in the classroom of 20 people at the same time

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