There already are gethostbyaddr() and gethostbyname() ... But where are the functions mentioned in the subject. Do I have to parse /etc/services to do that..?
How about:

SYNOPSIS
  mixed getservicebyname(string name)
  string getservicebyport(int port)

DESCRIPTION
  getservicebyname("ftp") would return "21"
  getservicebyport(80) would return "www"

Of course, when local /etc/services file has a mapping for it, otherwise it should return values like e.g. gethostbyaddr(..)
  getservicebyport(123456) would return "123456", when not defined in /etc/services

just my two cents
  Christian