На этот вариант выдало такую ошибку
<?php
$str = file_get_contents('php://input');
$data = json_decode($str);
header('Content-Type: application/json; charset=utf-8');
echo json_encode($data);
?>
Warning: Cannot modify header information - headers already sent by (output started at W:\home\korzinka\www\liqpay\liqpay.php:1) in W:\home\korzinka\www\liqpay\liqpay.php on line 6