Building query string programmatically in Guzzle?

Guzzle 6 - you could use query option param

// Send a GET request to /get?foo=bar
$client->request('GET', '/get', ['query' => ['foo' => 'bar']]);

http://docs.guzzlephp.org/en/stable/request-options.html#query

You can:

$c = new Client('http://test.com/api/1.0/function');

$request = $c->get();

$q = $request->getQuery();

$q->set('f', 4);
$q->set('l', 2);
$q->set('p', 3);
$q->set('u', 5);