PHP Threads do not seem to be working

I am making a Promise like in JavaScript, and I decided to do it using threads. So, I have the following class:

class Promise extends Thread {

    protected $action = null;
    protected $then = null;
    protected $catch = null;

    public function __construct(callable $callback){
        $this->action = $callback;
        $this->start();
    }

    public function run(){
        call_user_func_array($this->action, [
            function($result){
                if($this->then !== null){
                    call_user_func($this->then, $result);
                }
            },
            function($result){
                if($this->catch !== null){
                    call_user_func($this->catch, $result);
                }
            }
        ]);
    }

    public function then(callable $callback){
        $this->then = $callback;
        return $this;
    }

    public function catch(callable $callback){
        $this->catch = $callback;
        return $this;
    }

}

I then run the following code to test it:

// Run the first thread
(new Promise(function($resolve, $reject){
    sleep(5);
    $resolve(true);
}))->then(function(){
    echo 'Resolved 5 second promise' . PHP_EOL;
});

// Run the second thread
(new Promise(function($resolve, $reject){
    sleep(2);
    $resolve(true);
}))->then(function($result){
    echo 'Resolved 2 second promise' . PHP_EOL;
});

// Show a status
echo 'Waiting for Promises' to complete' . PHP_EOL;

When running this, I was expecting the following output:

Waiting for Promises' to complete
Resolved 2 second promise
Resolved 5 second promise

I instead get the following output:

Resolved 5 second promise
Resolved 2 second promise
Waiting for Promises' to complete

So, why is it not running in a thread like fashion?

Edit

I noticed that if I assign the Promise to a variable it outputs correctly, but feel I shouldn't have to do that...