User:/$ mkdir -g foo
User:/foo/$

mkdir() {
 if "$1" -eq "-g"
  mkdir "$2"
  cd "$2"
 else
  ## do the regular code
 fi
}

mkcd() {
 if "$1" -eq "-g"
  mkdir "$2"
  cd "$2"
 else
  ## do the regular code
 fi
}

mkdir () {
  local do_cd=
  if [ "$1" = "-g"]; then
    do_cd=1
    shift
  fi
  command mkdir "$@" &&
  if [ -n "$do_cd" ]; then
    eval "cd "${$#}""
  fi
}