The note by jm at guide42 dot com is inaccurate. Generator::getReturn() does not check the existence of the return statement, but rather whether the function has returned (finished executing).

<?php
function f(){
  yield;
}

f()->getReturn(); // Exception: Cannot get return value of a generator that hasn't returned

$f = f();
$f->next(); // NULL
$f->getReturn(); // NULL, because having no return statement implies an empty `return;`, and the void return value is resolved to null.

Just to be clear, the value returned by a generator is not yielded while it is running, but is available afterwards through the getReturn() method.

One use for this could be to record the output of a generator so that it can be iterated over again without having to reconstruct and re-run the generator. (Indeed, one could write a generator that simply does exactly this for any given iterable.)

<?php
function generate_squares($n)
{
    $record = [];
    for($i = 0; $i <= $n; ++$i)
    {
        yield ($record[] = $i * $i);
    }
    return $record;
}

$squares = generate_squares(10);
foreach($squares as $s)
{
    echo $s, ' ';
}
$recorded = $squares->getReturn();
echo "\nThat's [", join(', ', $recorded), "]";
?>

Recording keys as well would take a bit more work, as generators are free to repeat them but arrays aren't.

I'm not quite sure -- I feel there is an ambiguity issue here with returning a value inside a generator and ->getReturn() method of such generator as I have been able to return an array of sorts, something like this:

function dequeue(){
try{
  foreach($this->buffer->data as $data){ ... }
}
  return $statistics;
}

foreach(($generator = $this->dequeue()) as $bdata){
.....
}

if($statistics = $generator->getReturn()){
 // generator returns $statistics
}

Obviously this is not a complete example, but $statistics does become available with the returned array.