Return ACF Field Groups In JSON If You Only Have PHP. Revised code from this post David Egan - https://dev-notes.eu/2017/01/convert-acf-fields-registered-by-php-to-importable-json-format/

acf-php-fields-to-json.php

<?php
include_once '../wp-load.php';

$groups = acf_get_local_field_groups();
$json = [];

foreach ($groups as $group) {
  // Fetch the fields for the given group key
  $fields = acf_get_fields($group['key']);

  // Remove unnecessary key value pair with key "ID"
  unset($group['ID']);


  $new_fields = [];
  foreach($fields as $field) {
    // Remove unnecessary key value pair with key "ID"
    unset($field['ID']);
    $new_fields[] = $field;
  }

  // Add the fields as an array to the group
  $group['fields'] = $new_fields;

  // Add this group to the main array
  $json[] = $group;
}
//
$json = json_encode($json, JSON_PRETTY_PRINT);
// Optional - echo the JSON data to the page
echo "<pre>";
echo $json;
echo "</pre>";