yield-from-example.php

<?php

function myHttpHandler(Request $request, Response $response) {
    // async function that returns a promise
    // we use yield to wait for that promise to resolve then resume here
    // if there's some kind of error it will be thrown into our generator
    $session = yield loadSessionFromRequest($request);
    
    if ($session->hasValue('isLoggedIn')) {
        // pass the individual promises from generateHttpBody() through using `yield from`
        // when generateHttpBody() finishes we get its return result in $body
        $rawHtmlBody = yield from generateHttpBody($request);
    } else {
        $response->setStatus(301);
        $response->setHeader("Location", "http://domain.com/login");
        $rawHtmlBody = '';
    }

    $response->send($rawHtmlBody);
}

function generateHttpBody(Request $request, Session $session) {
    // returns a promise; pause here until the promise resolves.
    // this promise is passed through transparently inside myHttpHandler()
    // because we used `yield from` there.
    $user = yield loadUserFromDatabase($session->getValue('userId'));
    
    // Another promise -- again, this is passed through transparently
    // because we used `yield from` in myHttpHandler()
    $template = yield loadTemplateFromRedis($request->getUriPath());
    
    // ... do some stuff here to set template values ... //
    
    return $rawHtmlBody;
}

// And above this layer in the HTTP server code:
// $promise = coroutine(myHttpHandler($request, $response));
// $promise->when(...); // we know the response completed here