Random function on basic spells

[cacbo]

I want to simulate fight between heros in php. Some heros might have critical, bash, and other spells that have % to activate. So, I thought to make that like this:

if spell has 27% to activate i will put 27 numbers 1 (or any unique thing) on random indexes of array which have 100 members. All others members are any other numbers but not 1. Then i will get 1 random number between 1 - 100 and if on that index on array is nubmer 1 spell is activated. If not, do nothing.
This should work fine, but im interested how much php random function differs from w3 engine random function? Will it be simulated right?