wordpress() {
    local drt="$1"
    local domain="$2"
    local dbpass="$3"

    # should do more checking here, this is the bare minimum.
    # it avoids running `rm -rf /` if both $drt and $domain
    # are empty, and running `rm -rf "$drt/"` if $domain is empty.
    [ -z "$drt"    ] && error 1 '$drt is empty. aborting!'
    [ -z "$domain" ] && error 1 '$domain is empty. aborting!'
    [ -z "$dbpass" ] && error 1 '$dbpass is empty. aborting!'

    # should check exit status of each command run here and take
    # appropriate action on any failures.  Figure out what you want
    # to happen if any of the following fail and implement it.
    rm -rf "$drt"/"$domain"/ 2>/dev/null
    wp core download --path="$drt"/"$domain"/ --allow-root
    wp config create --path="$drt"/"$domain"/ --dbname="$domain"
      --dbuser="$domain" --dbpass="$dbpass" --dbhost=localhost
      --allow-root
}

error() {
  local ec msg

  ec="$1" ; shift
  msg="$*"

  [ -n "$msg" ] && echo "$msg" >&2
  # don't exit if $ec==0 - just return after printing a warning to stderr
  [ "$ec" != 0 ] && exit "$ec"
}

function1 () { ... ; }
function2 () { ... ; }
function3 () { ... ; }
function4 () { ... ; }

main () {
  function1
  function2
  function3
  function4
}

main

case "$var" in
  1) function1 ;;
  2) function2 ;;
  3) function3 ;;
  4) function4 ;;
  *) error 1 'Unknown case' ;;
esac