does php loads at most one php.ini, or are settings from higher-level php.ini inherited by more specific php.ini?

does php loads at most one php.ini, or are settings from higher-level
php.ini inherited by more specific php.ini?

Does PHP interpreter loads first php.ini it finds (according to its search
algorithm) and stops. Or does it load entire php.ini hierarchy and merges
the settings?
So, if I need to overwrite single setting for a single web site - do I
need to specify ALL values, or do I only need to specify values that are
different from "higher-level" php.ini?