I got solution of my problem i was missing swimlaneId attribute. I am posting code here for future reference so it will help others.
Now sending data using object like following
$object = new stdClass();
$object->name = 'Testing Task';
$object->columnId = 'xxxxxxxxxxxxxxxxxxxxxxxxxxxx';
$object->swimlaneId = 'xxxxxxxxxxxxxxxxxxxxxxxxxxx';
$object->color = 'green';
$url = "https://kanbanflow.com/api/v1/tasks";
$curl = curl_init();
curl_setopt($curl, CURLOPT_URL, $url);
curl_setopt($curl, CURLOPT_SSL_VERIFYPEER, false);
curl_setopt($curl, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($curl, CURLOPT_HTTPHEADER, $headers);
curl_setopt($curl, CURLOPT_POST, true);
curl_setopt($curl, CURLOPT_POSTFIELDS, json_encode($object));
$response = curl_exec($curl);
Array of Json_encode is also working here
$data = json_encode(array('name'=>'Testing of api', 'columnId' =>"f648831061e111e3a41aa3dbd7a40406", 'color'=>'red', 'swimlaneId' => 'd0635fc061e711e3a41aa3dbd7a40406'));