HTTP/1.1 400 BAD REQUEST Error - php

Question

My coding is this
$number1= $_POST{'username'};
set_time_limit(0);
error_reporting(0);
$post = array(
'phone_number'=>$number1);
$url="https://www.pokketdeal.com/lokiapp/otp";
$headers[]='Content-Type: application/json';
$headers[]='Content-Length: 31';
// $headers[]='X-Mclient: 3';
// $headers[]='x-wap-profile: http://www.1066.cn/uaprof/prof/Micromax/Micromax_A177.xml';
// $headers[]='X-Requested-With: com.pokkt.app.pocketmoney';
$headers[]='Accept-Encoding: gzip,deflate';
$headers[]='Accept-Language: en-US';
$headers[]='Accept-Charset: utf-8, utf-16, *;q=0.7';
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, "$url");
curl_setopt($ch, CURLOPT_USERAGENT, "okhttp/2.3.0" );
curl_setopt($ch, CURLOPT_HEADER, 1);
curl_setopt($ch, CURLOPT_VERBOSE, 1);
curl_setopt($ch, CURLOPT_POST, 0);
curl_setopt($ch, CURLOPT_COOKIEFILE, $ckfile);
curl_setopt($ch, CURLOPT_COOKIEJAR, $ckfile);
curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($post));
curl_setopt($ch, CURLOPT_PROXY, $proxy);
curl_setopt($ch, CURLOPT_FOLLOWLOCATION, 1);
curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
$html = curl_exec($ch);
echo $html. "<br>";
when submitting data i am getting server response as
HTTP/1.1 400 BAD REQUEST Server: nginx/1.4.6 (Ubuntu) Date: Thu, 10 Dec 2015 12:16:40 GMT Content-Type: text/html Transfer-Encoding: chunked Connection: keep-alive X-Frame-Options: SAMEORIGIN
Bad Request (400)
also my content type is application/json but response is text/html.