if there's not a 'php.ini' file in your public_html directory, then just create one with the above contents.
NB: 'document root' is the generic name for the home directory of your website. On some servers, it's in public_html/, on others it's in www/ on still others, it's httpdocs/
'Document root' is just shorthand for the purpose of the directory -- which *precise* file directory it is depends on your webhosting setup.