I am running phpBB forum. Zenphoto gallery will be its subdomain. I want to auto-generate the Zenphoto user when a user registers at phpBB. So that users do not have to go through double registration hassle. Is there any ZenphotoAPI exists for the registeration and login? Thanks

You would need to create a plugin for external authentification process. See the comments in zp-core/external_auth.php.
Or you create a complete custom authorization handler. See the comments within zp-core/lib-auth.php.