Do local php.ini files overwrite all variables on a central server php.ini?

I have a question about php.ini files. I am using a shared hosting server for hosting a website. They use a customized php.ini file for the server. I would like to override certain variables on a local directory. What I have heard is that I must include all the variables on the php.ini file or else it will set the unlisted ones to defaults. Is this true? Thanks!