access to php.ini

[unicorn]

PRODUCT: Plesk Control Panel.
Version: 8.6.0.5
OPERATING SYSTEM: Microsoft Windows 5.2
ARCHITECTURE: what is it?

PROBLEM:
I had tries diffrent ways to override php.ini settings but with no results.
I'm using PHP5 and I tried:

ini_set('upload_max_filesize', '12M');
ini_set('post_max_size', '12M');
ini_set('max_input_time', 300);
ini_set('max_execution_time', 300);

And creating a php.ini file if httpdocs with:
upload_max_filesize 12M

but in both cases i get that the "upload_max_filesize" value is 16M
meaning I mase no changes.

what am I doing wrong?

BTW: there is no forum for version 8 so I wrote here, hope I'll get a response