The manual's sections about fibers and their use cases are not very good. I searched online, and the examples don't really answer the title question.

For example, I understand fibers are stackful, where generators are stackless. So one can trivially write code like this:

function suspend(string $msg): int { $val = Fiber::suspend($msg); return $val; } $f1 = new Fiber(function() { $val = suspend('hey'); echo "f1 got $val\n"; }); $f2 = new Fiber(function() { $val = suspend('you'); echo "f2 got $val\n"; }); $res = $f1->start(); echo "\n$res\n"; $res = $f2->start(); echo "$res\n"; $f1->resume(100); $f2->resume(10); 

Now, this showcases the stackful nature of fibers very well: they can be suspended anywhere in the call stack! I would think that this would be hard to simulate with generators, and with plain generators I think it really is.

However, by leveraging generator delegation the code turns out to be equivalent and not so bad:

function suspend(string $msg): Generator { $parm = yield $msg; return $parm; } $g1 = (function() { $val = yield from suspend("hey"); echo "g1 got $val\n"; })(); $g2 = (function() { $val = yield from suspend("you"); echo "g2 got $val\n"; })(); $res = $g1->current(); echo "\n$res\n"; $res = $g2->current(); echo "$res\n"; $g1->send(100); $g2->send(10); 

The only difference is the return type of suspend: Functions that work with fibers are free to keep their original return type, while those that do a similar job with generators must return a generator.

So, I really want to know: What's the use case that fibers make possible or much less painful to write/mantain?

Thanks.