Generator::send

Reading the example, it is a bit difficult to understand what exactly to do with this. The example below is a simple example of what you can do this.

<?php
function nums() {
    for ($i = 0; $i < 5; ++$i) {
                //get a value from the caller
        $cmd = (yield $i);
        
        if($cmd == 'stop')
            return;//exit the function
        }     
}

$gen = nums();
foreach($gen as $v)
{
    if($v == 3)//we are satisfied
        $gen->send('stop');
    
    echo "{$v}\n";
}

//Output
0
1
2
3
?>

If you want to use generator::send() within a foreach loop, you will most likely get an unexpected result. The Generator::send() method resumes the generator, which means the pointer within the generator is moved to the next element in the generator list.

Here is an example:

<?php

class ApiDummy
{
    private static $apiDummyData = ['a', 'b', 'c', 'd', 'e'];

    public static function getAll(): Generator {
        foreach (self::$apiDummyData as $entry) {
            echo 'yielding $elem' . PHP_EOL;
            $newElem = (yield $entry);
            echo 'yield return: ' . $newElem . PHP_EOL;
        }
    }
}

$generator = ApiDummy::getAll();

// example iteration one with unexpected result
foreach ($generator as $elem) {
    echo 'value from generator: ' . $elem . PHP_EOL;
    $generator->send($elem . '+');
}

// example iteration two with the expected result
while ($generator->valid()) {
    $elem = $generator->current();
    echo 'value from generator: ' . $elem . PHP_EOL;
    $generator->send($elem . '+');
}
?>

The result of example iteration one:
yielding $elem
value from generator: a
yield return: a+
yielding $elem
yield return:
yielding $elem
value from generator: c
yield return: c+
yielding $elem
yield return:
yielding $elem
value from generator: e
yield return: e+

As you can see, the values b and d are not printed out and also not extended by the + sign.
The foreach loop receives the first yield and the send call causes a second yield within the first loop. Therefor the second loop already receives the third yield and so on.

To avoid this, one solution could be to use a while loop and the Generator::send() method to move the generator cursor forward and the Generator::current() method to retrieve the current value. The loop can be controlled with the Generator::valid() method which returns false, if the generator has finished. See example iterator two. 

The expected result of example iteration two:
yielding $elem
value from generator: a
yield return: a+
yielding $elem
value from generator: b
yield return: b+
yielding $elem
value from generator: c
yield return: c+
yielding $elem
value from generator: d
yield return: d+
yielding $elem
value from generator: e
yield return: e+

As of 7.3, the behavior of a generator in a foreach loop depends on whether or not it expects to receive data. Relevant if you are experiencing "skips".

<?php
class X implements IteratorAggregate {
    public function getIterator(){
        yield from [1,2,3,4,5];
    }
    public function getGenerator(){
        foreach ($this as $j => $each){
            echo "getGenerator(): yielding: {$j} => {$each}\n";
            $val = (yield $j => $each);
            yield; // ignore foreach's next()
            echo "getGenerator(): received: {$j} => {$val}\n";
        }
    }
}
$x = new X;

foreach ($x as $i => $val){
    echo "getIterator(): {$i} => {$val}\n";
}
echo "\n";

$gen = $x->getGenerator();
foreach ($gen as $j => $val){
    echo "getGenerator(): sending:  {$j} => {$val}\n";
    $gen->send($val);
}
?>

getIterator(): 0 => 1
getIterator(): 1 => 2
getIterator(): 2 => 3
getIterator(): 3 => 4
getIterator(): 4 => 5

getGenerator(): yielding: 0 => 1
getGenerator(): sending:  0 => 1
getGenerator(): received: 0 => 1
getGenerator(): yielding: 1 => 2
getGenerator(): sending:  1 => 2
getGenerator(): received: 1 => 2
getGenerator(): yielding: 2 => 3
getGenerator(): sending:  2 => 3
getGenerator(): received: 2 => 3
getGenerator(): yielding: 3 => 4
getGenerator(): sending:  3 => 4
getGenerator(): received: 3 => 4
getGenerator(): yielding: 4 => 5
getGenerator(): sending:  4 => 5
getGenerator(): received: 4 => 5

<?php
function foo() {
    $string = yield;
    echo $string;
    for ($i = 1; $i <= 3; $i++) {
        yield $i;
    }
}

$generator = foo();
$generator->send('Hello world!');
foreach ($generator as $value) echo "$value\n";
?>

This code falls with the error:
PHP Fatal error:  Uncaught exception 'Exception' with message 'Cannot rewind a generator that was already run'.
foreach internally calls rewind, you should remember this!