Convert ACF Fields Registered by PHP to Importable JSON Format

convertor.php

<?php 
/**
 * create a file acf-import.json inside your theme main folder
 * use this code in themes functions.php or in any custom plugin. 
 * It is one time code so it will add all fields json entry inside acf-import.json file
 */
add_action('init', 'create_fieldjson_from_fieldphp');
function create_fieldjson_from_fieldphp() {
    $groups = acf_get_local_field_groups();
    $json = [];

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

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

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

    // Write output to file for easy import into ACF.
    // The file must be writable by the server process. In this case, the file is located in
    // the current theme directory.
    $file = get_template_directory() . '/acf-import.json';
    file_put_contents($file, $json );
}