Depends, actually. Depends on how you use php inside that webserver:
- using php as module inside the http server: you have to restart the http server process
- using php as cgi backend: you do not have to restart the http server process or anything else
- using php fastcgi: you have to restart the fastcgi daemon, not the http server
- using php-fpm: you have to restart the fpm server process, not the http server process