CallbackRun dont run in parallel

[webysther]

Using the Readme example, tag and parallel execution don't work on CallbackRun.

[h-bragg]

Hi,

Good point, I guess it sort of goes against what the actual library is for; being able to run callbacks in parallel in PHP is... horrible, I'll see if can be done sensibly, but I expect not.

At the least I will update the documentation to indicate it doesn't run in parallel.