gistfile1.php

<?php

$obj = new \stdClass;
$obj->x = '';

$gen1 = function() use ($obj) {

    $obj->x .= ' 1 ';
    yield;
    $obj->x .= ' 2 ';
    yield;
    $obj->x .= ' 3 ';
    yield;
    $obj->x .= ' 4 ';
};

$gen2 = function() use ($obj) {

    $obj->x .= ' A ';
    yield;
    $obj->x .= ' B ';
    yield;
    $obj->x .= ' C ';
    yield;
    $obj->x .= ' D ';
};

$master = new TaskMaster;
$master->queue($gen1());
$master->queue($gen2());
$master->run();

echo trim($obj->x); // '1 A 2 B 3 C 4 D'

class TaskMaster
{
    protected $tasks = [];

    public function queue(\Generator $generator)
    {
        $task = new Task($generator);
        $this->schedule($task);
    }

    public function schedule(Task $task)
    {
        $this->tasks[] = $task;
    }

    public function run()
    {
        while (count($this->tasks)) {

            $task = array_shift($this->tasks);
            $task->run();

            if ( ! $task->isDone()) {
                $this->schedule($task);
            }
        }
    }
}

class Task
{
    protected $generator;

    protected $initiated = false;

    protected $sendValue;

    public function __construct(\Generator $generator)
    {
        $this->generator = $generator;
    }

    public function setSendValue($value)
    {
        $this->sendValue = $value;
    }

    public function run()
    {
        if ( ! $this->initiated) {
            $this->initiated = true;
            return $this->generator->current();
        }

        return $this->generator->send($this->sendValue);
    }

    public function isDone()
    {
        return ! $this->generator->valid();
    }
}