where to put php.ini exactly ?

Good morning everybody ! 

I am facing a problem with a custom php.ini, here is the point:

I am hosted at 1&1 on a shared server, with 64M of max_upload_filesize and max_execution_time at 50000.
I have a controller with an upload function in standard PHP which calls (once uploaded) another private function into the same controller to end the job.

It seems to me that the php.ini values are too short, because over 35M, the files are not uploaded.  I tried to append some php_value to the htaccess and got an artistic error 500. Then, I put a personnal  PHP.ini file with only 

PHP Code:

upload_max_filesize = 200M
post_max_size = 200M 


 inside to see what happens, and here is the problem : 
when I put the ini file inside the controller's folder, nothing happens and a ini_get() returns 64M and when I put it inside the root, I have a full white screen.
I have called 1&1 and the guys confirmed that I can override the php.ini by placing this file in the root. But as told, it gives me this infamous white screen 

Is there any particular reason to get this white screen, or should I modify something in the CI configuration ? Or is there anything to see with the custom script and not the CI upload library ?

I read in another thread that the CI ini_set() was not efficient, so there is just the php.ini to solve that I think.

If anybody can advise me, it would be great

Thanks in advance

Guillaume