an example async and promise simulate

async.php

<?php

class Promise
{
    private array $callbacks = [];
    private array $rejectedCallbacks = [];

    /**
     * @param Closure $closure
     */
    public function __construct(private readonly Closure $closure)
    {
        ($this->closure)($this->resolve(...), $this->reject(...));
    }

    /**
     * @param $value
     * @return void
     */
    public function resolve($value): void
    {
        foreach ($this->callbacks as $callback) {
            $callback($value);
        }
    }

    /**
     * @param $reason
     * @return void
     */
    public function reject($reason): void
    {
        foreach ($this->rejectedCallbacks as $callback) {
            $callback($reason);
        }
    }

    public function catch(callable $callback): self
    {
        $this->rejectedCallbacks[] = $callback;
        return $this;
    }

    /**
     * @param callable $onFulfilled
     * @return Promise
     */
    public function then(callable $onFulfilled): self
    {
        $this->callbacks[] = $onFulfilled;
        return $this;
    }
}

class Loop
{
    private array $callbacks = [];
    private static ?self $loop = null;

    /**
     * @param callable $callback
     * @return void
     */
    public function addCallback(callable $callback): void
    {
        $this->callbacks[] = $callback;
    }

    /**
     * @return void
     */
    public function run(): void
    {
        while ($callback = array_shift($this->callbacks)) {
            $callback();
        }
    }

    public static function new(): self
    {
        return static::$loop ??= new static();
    }
}

function async(Closure $closure): Promise
{
    return new Promise(function ($resolve, $reject) use ($closure) {
        Loop::new()->addCallback(function () use ($resolve, $reject, $closure) {
            try {
                $closure($resolve, $reject);
            } catch (\Throwable $e) {
                $reject($e->getMessage());
            }
        });
    });
}

async(static function ($resolve) {
    return $resolve('success');
})->then(function ($value) {
    echo $value . PHP_EOL;
})->catch(function ($reason) {
    echo 'Error: ' . $reason . PHP_EOL;
});


async(static function ($resolve, $reject) {
    if ((int)date('Y') === 2201) {
        return $resolve('happy holidays!');
    }
    return $reject('failed');
})->then(function ($value) {
    echo $value . PHP_EOL;
})->catch(function ($error) {
    echo 'Error: ' . $error . PHP_EOL;
});

Loop::new()->run();