randomly subtract desired percentage from a array using php

I have a array which contains 40 elements like this

$i = array('1','2','3','4','5','6','7','8','9','10','11',
'12','13','14','15','16','17','18','19','20','21','22','23','24',
'25','26','27','28','29','30','31','32','33','34','35',
'36','37','38','39','40');

now i have 4 variations like 5%, 10%, 15%, 20%

The requirement is i need to take array values randomly after subtracting the desired variation.

Lets say i used 5% variation so i need to separate 5% from the array ie 2 elements i need to remove randomly from the array and keep the rest 38 element in a new array randomly.so both the resulted array as well subtracted elements need to be in two different array.

I need a function with two parameter ie one is variation and another is required array ie resultant array or subtracted elements array.

This same sequence follows to all other variations.