Cannot modify header information – headers already sent

Okay I want to share with you guys in case you have the same problem.

I had warning cannot modify header so I unable to open media library and even login back, I spent whole day to fix it.
It said the error was found on the first line so it was pointing to php open tag, I’ve checked it but my first line was just okay, no white space no other unnecessary code.
My open tag is totally fine.
I talked to my Host customer service and he solved it in a matter of seconds.
What he did was a quick custom .user.ini alteration. https://kb.site5.com/php/how-to-repair-headers-already-sent-php-errors/

Finally I was able to smile again:)