PHPUnit - one coverage report from multiple project folders?

I have a project that is structured like so:

project
├── app
│   ├── phpunit.xml
│   ├── src
│   └── tests
│       └── unit
└── modules
    ├── module1
    │   ├── phpunit.xml
    │   ├── src
    │   └── tests
    │       └── unit
    └── module2
        ├── phpunit.xml
        ├── src
        └── tests
            └── unit

All of the core interfaces/classes are in app/src, and all of the extensions/implementations are in modules/**. Thus, the src folders contain all the Controllers, Models, etc., and the adjacent tests/unit folder contains all of the unit tests for these objects.

I'm trying to generate one coverage.xml report (and more importantly an HTML report) that contains the coverage results from all of the tests across both app/ and modules/. Can anyone suggest the best way to accomplish this?