In order to change speciffic settings in the global php.ini configuration file you should first be aware that such changes are not allowed on our shared hosting environment due the fact that these are global changes which will affect all of the users which are located ont he same server as you are.
However there is an option which you should consider. Into your public_html folder usually is located a file called .htaccess. You can insert in it the settings you need for your web-site and they will be considered on local level. In other words these settings will affect only your web-site and not other users.
As for our dedicated services users (VPS/Dedicated server ) if they have the root password they can edit their global php.ini file without any issues due the fact that they are using services which does not involves shared hosting environment with other users and such changes wont affect the other users.