problems getting curl / json work with php - php

Question

I just wanted to ask one of my servers (let's call him mysql-server) a simple question to get some result out of a MySQL database.
This mysql-server provides the result as json decoded array in the form - e.g. like this:
$data = array("first_name" => "First name","last_name" => "last name");
$data_string = json_encode(array("customer" => $data));
echo $data_string;
which works pretty well and returns something like this:
{"customer":{"first_name":"First name","last_name":"last name"}}
Just to check that everything is fine I added a few lines:
echo "<pre>";
print_r(json_decode($data_string, true))."</pre>";
and got the following:
Array(
[customer] => Array
(
[first_name] => First name
[last_name] => last name
)
)
So everything seems to be fine on this server side.
If I now call the url of that script from the second-server using:
$data_string = json_encode(array('customer' => $customerNo));
$curlHandler = curl_init($url);
curl_setopt_array($curlHandler, [
CURLOPT_URL => $url,
CURLOPT_POST => true,
CURLOPT_POSTFIELDS => $data_string,
CURLOPT_RETURNTRANSFER => true,
CURLOPT_VERBOSE => true,
CURLOPT_STDERR => fopen($fp, 'w'),
//CURLOPT_SSL_VERIFYHOST == 2,
//CURLOPT_VERIFYPEER == true,
CURLOPT_HTTPHEADER => array(
'Accept: application/json',
'Content-Type: application/json',
'Content-Length: ' . strlen($data_string))
]);
$response = curl_exec($curlHandler);
var_dump(json_decode($response, true));
I got NULL.
Instead of using
echo $response;
brings the following result to the screen:
{"customer":{"first_name":"First name","last_name":"last name"}}
and a
var_dump($response);
brings the following to the screen:
string(268) " {"customer":{"first_name":"First name","last_name":"last name"}} "
So the data is transfered back and stored in the variable $response as it should be, but may anyone give me please a hint, why I can't access it using json_decode?
Many thanks and best
Paul