<?php
$postData = file_get_contents('php://input');
$file = 'post_log.txt';
file_put_contents($file, $postData . PHP_EOL, FILE_APPEND);

$dataToSave = json_encode($_POST, JSON_UNESCAPED_UNICODE);
file_put_contents('log.txt', $dataToSave . PHP_EOL, FILE_APPEND);