My suggested milestones:
Novice: You’ve used variables, conditions, loops, functions, etc.
Intermediate: You’ve used OOP, namespaces, exceptions, autoloading, testing, etc; and familiar with security issues, coding styles, etc.
Expert: You’ve used the larger PHP ecosystem, such as libraries (Symfony components, Doctrine, Guzzle, Markdown, etc), frameworks (Laravel, Symfony full stack, CodeIgniter, Cake, Yii, etc), and tools (Composer, CodeSniffer, XDebug, etc); and familiar with design patterns (MVC, decorator, strategy, DI, etc), and other concepts such as SOLID, composition v inheritance, etc.
Guru: You’ve made significant contributions to the PHP community and ecosystem. This is the category I would imagine people like Fabien Potencier would fall into, with things like Symfony, Composer, and Twig.
God like: You’ve worked on an execution engine for PHP. This is the category I would imagine the developers for PHP7 or HHVM would fall into. (Ironically, these guys probably work in C++ on a daily basis rather than PHP.)