json exchange between PHP scripts on different servers

Question

PHP

Greetings!
Task: On one server there is a folder with .txt files, they contain pure JSON. I read data from these files with the client.php script located next to the folder, now I need to send the read data to another server. The server.php script is located on the remote server, which should receive this data, give a response to the client.php script, process it and save it to a .txt file in JSON format.
I read files from client.php without problems. From server.php to write to a file - also ok. The problem is in the middle. I can’t send data in any way, apparently, they don’t even reach server.php. What am I doing wrong?
client.php:

<?php
$api_key = 'B12347162936387BB33534B85C873'; // Для простой проверки
$api_url = 'http://remote.dev/server.php'; // Куда отправлять
$file = '20140525/data.txt'; // Локальный файл с json-данными
$json_data = file_get_contents( $file ); // Читаем из файла
// Пробуем отправить, для начала только $api_key
$curl = curl_init($api_url);
curl_setopt($curl, CURLOPT_CUSTOMREQUEST, "POST");
curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
curl_setopt($curl, CURLOPT_HEADER, 0);
curl_setopt($curl, CURLOPT_POSTFIELDS, json_encode($api_key));
curl_setopt($curl, CURLOPT_HTTPHEADER, array(
                                            'Content-Type:application/json',
                                            'Content-Length:'.strlen($api_key)
                                        ));
$json_response = curl_exec($curl);
$curl_errorno = curl_errno($curl);
$status = curl_getinfo($curl, CURLINFO_HTTP_CODE);
echo $json_response; // Выводим ответ
curl_close($curl);
?>

server.php:

<?php
$api_key = 'B12347162936387BB33534B85C873'; // Для сверки
print_r ($_POST); // Смотрим прилетели ли данные?
print_r ($_REQUEST); // Может сюда прилетели?
?>

Received response - 2 empty arrays:
Array ( ) Array ( )
What's wrong? I've been banging my head against the wall for an hour now. Google did not help, documentation on curl_setopt () too. I tried to send different variations - $api_key, $data, collected them into one array, translated them into json, and so on. The answer is the same, empty arrays.

Answer 1

It is necessary to pass an array or receive raw data - $HTTP_RAW_POST_DATAorphp://input

Answer 2

I think the problem is that you do not explicitly specify in which field to transfer data. Try the following construction:

curl_setopt($curl, CURLOPT_POSTFIELDS, array(
    'apikey' => $api_key,
    'data' => $json_data,
));

You can also try specifying the entire file to transfer at once.
Original: www.php.net/manual/en/function.curl-setopt.php