ClosureTaskとGeneratorを使用したタイマー

timer.php

<?php

declare(strict_types=1);

use pocketmine\plugin\Plugin;
use pocketmine\scheduler\ClosureTask;
use pocketmine\scheduler\TaskHandler;
use Generator;

$generator = (function (int $second): Generator {
    for ($i = $second; $i >= 0; $i--) {
        yield;
        if ($i > 0) {
            $this->getLogger()->info("残り " . $i . " 秒");
        } else {
            $this->getLogger()->info("カウント終了！");
        }
    }
})(5);
$task = new ClosureTask(function (int $currentTick) use (&$handler, $generator): void {
    /** @var TaskHandler $handler */
    /** @var Generator $generator */
    if ($generator->valid()) {
        $generator->next();
    } else {
        $handler->cancel();
    }
});
/** @var Plugin $this */
$handler = $this->getScheduler()->scheduleRepeatingTask($task, 20);