I found from other discussions that USERENV('SESSIONID') is used for last_update_login
but still searching for
There is fnd_global.login_id, fnd_global.user_id (no need for to_number). But if you are in the back-end without logging in, there is no user. You could use user 0 (sysadmin) or -1 (anonymous). It makes sense that you would use that, since any user you would use is actually not true right (that user is not currently logged in).
Or maybe you can use a default user, like a WEBSERVICE_DEFAULT_USER or something and login as that user before you perform actions (using fnd_global.apps_initialize).
Many thanks, that's correct, I used fnd_global.user_id it worked fine