php.ini is output_buffering effected after Global Variables change?

I am putting together a site based on iSchoolAd a apache/php/mysql script

It is being developed locally using PHPDEV.

After completing modifications the script worked fine.

Till I changed global_variables to on in php.ini (to make a old script
run).

When I went back to my project I received a message when trying to login:
Warning: unable to add to header already.........

So I changed global_variables back to off. Rebooted and guess what
Same message when I tried to run my script

After much searching I found a reference to changing output_buffering to on
as a fix for the warning. And it cured the problem.

My question is why did it work before I changed the global variables to on
(I understand, I think, how that caused a problem) And not work after I
changed it back to Off, until I changed output_buffering to on? I never
touched that in the first place.

-- 
God Bless you,
Al Kolff