JSON increment doesn't work - php

Question

I am working on a JSON file in which I want to increment instead of overwriting the data. However I cannot seem to do this (since it keeps on overwriting instead of adding data with an incremented ID).
The code underneath is: database_json.php (as u can see I include it in saveJson.php)
$databaseFile = file_get_contents('json_files/database.json');
$databaseJson = json_decode($databaseFile, true);
$database = $databaseJson['data'];
the below is the code of the file saveJson.php contains the following code:
// below starts a new page, the page that submits the form called saveJson.php
include_once('database_json.php');
$data = $_POST;
//Setup an empty array.
$errors = array();
if (isset($data)) {
$newExerciseData = $data;
$exerciseArray = $data['main_object'];
$databaseFile = 'json_files/database.json';
$textContent = file_get_contents($databaseFile);
$database = json_decode($textContent, true);
if ($data['id'] === 'new') {
if (count($database['data']) == 0) {
$ID = 0;
}
else {
$maxID = max($database['data']);
$ID = ++$maxID["id"];
}
$newJsonFile = 'jsonData_' . $ID . '.json';
$newJsonFilePath = 'json_files/' . $newJsonFile;
//Create new database exercise_txt
$newArrayData = array(
'id' => $ID,
// a lot of variables that aren't related to the problem
);
$database['data'][] = $newArrayData;
file_put_contents($databaseFile, json_encode($database, JSON_UNESCAPED_UNICODE, JSON_PRETTY_PRINT));
file_put_contents($newJsonFilePath, json_encode($newExerciseData, JSON_UNESCAPED_UNICODE, JSON_PRETTY_PRINT));
}
else {
$index = array_search((int) $_POST['id'], array_column($database['data'], 'id'));
$correctJsonFile = 'json_files/jsonData_' . $_POST['id'] . '.json';
$newJsonFile = 'jsonData_' . $_POST['id'] . '.json';
$newJsonFilePath = 'json_files/' . $newJsonFile;
//Create new database exercise_txt
$newArrayData2 = array(
'id' => (int) $_POST['id'],
// more not related to problem variables
);
$database['data'][$index] = $newArrayData2;
file_put_contents($databaseFile, json_encode($database, JSON_UNESCAPED_UNICODE));
file_put_contents($newJsonFilePath, json_encode($newExerciseData, JSON_UNESCAPED_UNICODE));
}
echo json_encode($newExerciseData, JSON_UNESCAPED_UNICODE);
}
so, what I wish for: To increment the IDs and NOT to overwrite the data.
I already did some research and didn't find any useful information besides this --> Auto increment id JSON, but to me it looks like I have the same principle applied.