I’ll give a couple of examples of saving the data of the incoming POST request to a file:
Option 1
$s = implode('|', $_POST); exec('echo $s >> file.txt');
Option 2
file_put_contents('file.txt', json_encode($_POST));
Example of option 2 with a specific parameter:
file_put_contents('file.txt', json_encode($_POST['parameter']));