yield

协程是一种轻量级的线程，可以在同一个线程中控制多个子程序的执行流程，从而避免了线程上下文切换的开销。这段代码使用PHP实现了简单的协程

function coroutine1() {
    for ($i = 0; $i < 5; $i++) {
        echo "coroutine1: " . $i . PHP_EOL;
        yield;
    }
}

function coroutine2() {
    for ($i = 0; $i < 10; $i++) {
        echo "coroutine2: " . $i . PHP_EOL;
        yield;
    }
}

$generator1 = coroutine1();
$generator2 = coroutine2();

while (true) {
    $generator1->current();
    $generator1->next();
    if (!$generator1->valid() && !$generator2->valid()) break;

    $generator2->current();
    $generator2->next();
    if (!$generator1->valid() && !$generator2->valid()) break;
}