如何在PHP中使用asana API获取不完整的任务

I have to fetch only incomplete task using Asana API

$projectId = 'XXXXXXXXXXX';
$args =  array('completed_since' => now);
$tasks = $asana->getProjectTasks($projectId, $args);    

$tasksJson = json_decode($tasks);
echo "<pre>"; print_r($tasksJson);

This code fetch all tasks (completed & incompleted tasks) related to that projectId. But I want to fetch seperate incomplete & completed task.