create user in moodle through web services php

I am trying to create a user through web services in Moodle by using PHP and I am using ws function: `core_user_create_users`. but it gives me an error. >{"exception":"webservice_access_exception","errorcode":"accessexception","message":"Access control exception"} i am using moodle ver3.4. Can you tell me how to add capabilities to the current token with respect to the user if its problem of web service function? if its other problem please tell me. Thanks,