How to post json data of multidimentional array using curl?

finally got the solution here it is. and it's working with multidimentional array.

$urltopost = "http://example.com/webservice/service.php";
$datatopost = array (0 =>array('a'=>'b','c'=>'d'),1 =>array('a'=>'b','c'=>'d'),2 =>array('a'=>'b','c'=>'d'),3 =>array('a'=>'b','c'=>'d'));
$post_data = array('data' => serialize($datatopost));

$ch = curl_init ($urltopost);
curl_setopt ($ch, CURLOPT_POST, true);
curl_setopt ($ch, CURLOPT_POSTFIELDS,$post_data);
curl_setopt ($ch, CURLOPT_RETURNTRANSFER, true);
$returndata = curl_exec ($ch);

echo "<pre>";
print_r(unserialize($returndata));

service.php code

$temp = unserialize($_POST['data']);
echo serialize($temp);

try something like this: (put your json string length into content-length)

$data = array("name" => "Hagrid", "age" => "36");                                                                    
$data_string = json_encode($data);                                                                                   

$ch = curl_init('http://api.local/rest/users');                                                                      
curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "POST");                                                                     
curl_setopt($ch, CURLOPT_POSTFIELDS, $data_string);                                                                  
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);                                                                      
curl_setopt($ch, CURLOPT_HTTPHEADER, array(                                                                          
    'Content-Type: application/json',                                                                                
    'Content-Length: ' . strlen($data_string))                                                                       
);                                                                                                                   

$result = curl_exec($ch);

You are trying to post raw JSON data on service.php.
Passing raw JSON data doesn't populate $_POST array. On service.php page you need to catch passed data from php stdin stream like shown below:

$fp = fopen("php://input", "r");
$data = stream_get_contents($fp);
$decoded_json_data = json_decode($data);

var_dump($decoded_json_data);

Also there is another approach which lets you to populate $_POST array:

    $urltopost = "http://example.com/webservice/service.php";
    $datatopost = array (0 =>array('a'=>'b','c'=>'d'),1 =>array('a'=>'b','c'=>'d'),2 =>array('a'=>'b','c'=>'d'),3 =>array('a'=>'b','c'=>'d'));
    $data_string = http_build_query(['json' => json_encode($datatopost)]);

    $ch = curl_init($urltopost);
    curl_setopt($ch, CURLOPT_HTTPHEADER, ['Content-Type: application/x-www-form-urlencoded',
  'Content-Length: '.strlen($data_string)]);
    curl_setopt($ch, CURLOPT_HEADER, true);
    curl_setopt($ch, CURLOPT_POST, true);
    curl_setopt($ch, CURLOPT_POSTFIELDS, $data_string);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
    $returndata = curl_exec($ch);

Now, on service.php page you can access your multidimensional array in the following way:

if (isset($_POST['json'])){
    $data = json_decode($_POST['json']);
}