Level 1
Can read, understand, and debug existing backend code. Can implement basic functionality within an existing site. Understands common PHP syntax and is comfortable reading through PHP code.
Level 2
Can implement a moderately complex route->controller->view feature within an existing MVC framework-based site (Laravel, Ruby on Rails, Yii, etc). Understands how to create simple PHPunit tests and how to modify existing ones.
Level 3
Can build clean new sites on at least one MVC framework, without help. Can architect and create PHPunit test coverage for a project. Can architect and build complex features, like an API, within an MVC framework. Can work with a JS backend.
Level 4
Has intimate knowledge of how common frameworks function. Is an expert in PHP syntax, best practices, and performance. Knows how to write extensive test coverage for an application. Understands JS backends strongly and can architect and write new applications in a framework like Express.
Level 5
Can work in more useful languages other than PHP and JS (Ruby, Go, Elixer, Python, etc.). Is a leading expert on backend coding methodologies and techniques.