It looks great. What are other existing tools? And how do they compare to them?

Sourcegraph Conc is broadly similar in providing pool helpers, but doesn't provide the same fine grained batching options: https://github.com/sourcegraph/conc

Uber CFF does code generation, and has more of a focus on readability and complex dependency chains: https://github.com/uber-go/cff

Thanks for sharing!

Practicality aside, there seems to be a lot we can learn from the implementation.

Great work. Are there any plans to convert some of the code to OSS?

PHP is slowly dying