toLazyPromise.php

<?php

use Interop\Async\Promise;
use Rx\Observable;

function toLazyPromise(Observable $observable) {
    return new class($observable) implements Promise {
        /** @var Promise */
        private $promise;

        private $sourceObservable;

        public function __construct(Observable $sourceObservable)
        {
            $this->sourceObservable = $sourceObservable;
        }

        public function when(callable $onResolved)
        {
            if (!$this->promise) {
                $this->promise = $this->sourceObservable->toPromise();
            }

            return $this->promise->when($onResolved);
        }
    };
}