by Mohammed Essaid MEZERREG.
We created our solution for this, a local plugin and added external service to it, one of them is for checking the existence of the user through it's more custom to our needs and what we are willing to achieve, but you can always try this solution because it's the ideal one till We have an official function for it.
The common thing between our service and yours is :
just put this inside your externallib.php file, for starter you can put it directly in externallib.php file under the /user root directory in moodle. And don't forget to declare it in services.php file, you can find it under /lib/db/services.php if you put the function in the user folder externallib.php file. Or the services.php under /db in your plugin.
