How to combine a Dart project with PHP?
You don't generate content or scripts in PHP to be displayed by Dart when using Dart. Dart is best used with Single Page Application Frameworks. (AngularDart, PolymerDart)
Idealy, you build the entire client code in Dart. Dart communicates with the server using HTTP requests or WebSocket to send or fetch the data. The data format is usually JSON but can also be any other format (images, audio, video, ...). Only data is sent between Dart client and PHP server.
When you build Dart to JavaScript the generated output can be integrated with your PHP like some other static content (HTML, Images, ...).