tricky timer in reactphp

testTimer-wrong.php

<?php

require_once 'vendor/autoload.php';

$loop = React\EventLoop\Factory::create();
$context = new React\ZMQ\Context($loop);

$timer = $loop->addPeriodicTimer(20, function() use($timer) {
    global $loop;
    echo "is executed\n";
    $loop->cancelTimer($timer);
});

$loop->run();

/* it fails with:

Fatal error: Uncaught TypeError: Argument 1 passed to React\EventLoop\ExtUvLoop::cancelTimer() must implement interface React\EventLoop\TimerInterface, null given, called in /home/scroll/code/tests/testTimer.php on line 11 and defined in /home/scroll/code/vendor/react/event-loop/src/ExtUvLoop.php:164
Stack trace:
#0 /..../testTimer.php(11): React\EventLoop\ExtUvLoop->cancelTimer(NULL)
#1 /..../vendor/react/event-loop/src/ExtUvLoop.php(146): {closure}(Object(React\EventLoop\Timer\Timer))
#2 [internal function]: React\EventLoop\ExtUvLoop->React\EventLoop\{closure}(Object(UVTimer))
#3 /..../vendor/react/event-loop/src/ExtUvLoop.php(230): uv_run(Object(UVLoop), 1)
#4 /..../testTimer.php(14): React\EventLoop\ExtUvLoop->run()
#5 {main}
  thrown in /..../vendor/react/event-loop/src/ExtUvLoop.php on line 164
/*

testTimer.php

<?php

require_once '../vendor/autoload.php';

$loop = React\EventLoop\Factory::create();
$context = new React\ZMQ\Context($loop);

$timer = $loop->addPeriodicTimer(20, function() use(&$timer) {
    global $loop;
    echo "is executed\n";
    $loop->cancelTimer($timer);
});

$loop->run();

// it print 'is executed' and exit cleanly
// (it exists because nothing more is attached to the $loop after cancel)

use keyword works the same as function parameters: it accepts parameters as value or as reference.
If passed as value, the only value available at that point was null (still uninitialized variable), in fact the $timer is returned by addPeriodicTimer() method after the call.

rif. https://blog.wyrihaximus.net/2015/01/reactphp-timers/