working with ini files

[Benjamin Adams <badams@xxxxxxxxxxxxxx>]

I have created my own ini file. I can read the values into an array  
fine. but What I want to do is change a value in the ini file. Example

file.ini
dog = 3
cat = 4
fish = 7

altered file.ini
dog = 3
cat = 5
fish = 7

how can I do this, I tried using ini_set but its not working.
help would be great, Thanks!!
Ben