If you go to Site administration > Server > PHP Info and look a few lines down you will find the php.ini location that the current PHP installation is using. Moodle doesn't access it directly (although it can read the settings via a PHP command) it's the config file for PHP itself.
Whether you can access that file to change it or (as is possible) there is some other method to override the settings is entirely down to how your host has configured things. So, you need to ask your hosting company.