Guzzle on_redirect无效

I'm using Guzzle I'm having troubles with on_redirect.

I have basic sample here:

$client = new Client([ 
'handler' => $stack, 
'allow_redirects' => [ 
            'on_redirect' => function () {echo '123';},         
             'track_redirects' => true, ], 
]);

And it's not working.

Redirect middleware is included.

Can somebody please explain me what am I doing wrong?