作为协程的yield（rvalue）的PHP文档

You can create a generator in PHP that yields values:

function genOneTwoThree() {
  foreach ([1,2,3] as $i => $number)
    yield $i => $number;
}

Then you can consume them like:

foreach (genOneTwoThree() as $i) {
  echo $i;
}

This "lvalue" usage is documented here: https://secure.php.net/manual/en/language.generators.syntax.php#control-structures.yield

But yield can also be used as an rvalue like this:

function printer() {
  while (true) {
    $string = yield;
    echo $string;
  }
}
$printer = printer();
$printer->send('Hello world!');
$printer->send('Bye world!');

This "consumer" example is given in the "Generator" documentation at https://secure.php.net/manual/en/generator.send.php

Is there another place that actually documents this type of usage? Some other specific question I have are:

• Can rvalue yield functions return something, and when does that happen?
• What happens when rvalue yield functions affect global state
• Can you use rvalue and lvalue yield to filter a generator?