the plugin didn't work because my server uses php.ini instead of php5.ini.
The documentation says :
3. If the plugin did not work, it may be because your server does not recognize php5.ini, and only recognizes php.ini. So, check the box labeled “USE php.ini INSTEAD OF php5.ini” and click the blue button once. Then, check your current status again after a few minutes.
but I didn't find the check box and the blue button. where are they ?