Create User and Responsibility from Back end

How to create user through back end?
How to assign responsibility to existing user through PL/SQL block?
APIs for creating user from back end and assign responsibility to it?

Create user through PL/SQL block:

DECLARE
  lc_user_name                 VARCHAR2(100) := 'DIKSHA.GAIKWAD';
  lc_user_password          VARCHAR2(100) := 'Oracle123';
  ld_user_start_date          DATE          := TO_DATE('31-AUG-2015');
  ld_user_end_date           VARCHAR2(100) := NULL;
  ld_password_date          VARCHAR2(100) := NULL;
  ld_password_lifespan_days NUMBER        := NULL;
  ln_person_id                       NUMBER        := NULL;
  lc_email_address                VARCHAR2(100) := NULL;
BEGIN

fnd_user_pkg.createuser
( x_user_name            => lc_user_name,
  x_owner                  => NULL,
  x_unencrypted_password => lc_user_password,
  x_start_date                     => ld_user_start_date,
  x_end_date                      => ld_user_end_date,
  x_password_date             => ld_password_date,
  x_password_lifespan_days => ld_password_lifespan_days,
  x_employee_id            => ln_person_id,
  x_email_address          => lc_email_address );
COMMIT;
EXCEPTION
WHEN OTHERS THEN
  ROLLBACK;
  DBMS_OUTPUT.PUT_LINE(SQLERRM);
END;
/
SHOW ERR;

Assign responsibility to User through PL/SQL block:

DECLARE
  v_user_name      VARCHAR2(30) := upper('DIKSHA.GAIKWAD');
  v_resp                VARCHAR2(30) :='System Administrator';
  v_resp_key        VARCHAR2(30);
  v_app_short_name VARCHAR2(50);
BEGIN

  SELECT r.responsibility_key , a.application_short_name
  INTO v_resp_key, v_app_short_name
  FROM fnd_responsibility_vl r, fnd_application_vl a
  WHERE r.application_id           =a.application_id
  AND upper(r.responsibility_name) = upper(v_resp);

  fnd_user_pkg.AddResp (
  username => v_user_name,
  resp_app => v_app_short_name,
  resp_key => v_resp_key,
  security_group => 'STANDARD',
  description => NULL,
  start_date => sysdate,
  end_date => NULL );

  COMMIT;

  DBMS_OUTPUT.put_line ('Responsibility:'||v_resp||' '||'is added to the User:'||v_user_name);
EXCEPTION
WHEN OTHERS THEN
  DBMS_OUTPUT.put_line ('Unable to add the responsibility due to'||SQLCODE||' '||SUBSTR(SQLERRM, 1, 100));
  ROLLBACK;
END;