convert from a multi-dimensional json object to a flat csv table

json2csv.php

<?php

/**
 * @param string $jsonFilename
 * @param string $csvFilename
 * @return bool|int
 */
function json2csv($jsonFilename, $csvFilename)
{
    $json = file_get_contents($jsonFilename);
    $data = json_decode($json, true);
    $fp = fopen($csvFilename, 'w');
    $keys = [];
    foreach ($data as $row) {
        foreach ($row as $key => $value) {
            $keys[] = $key;
        }
    }
    $header = array_unique($keys);
    $len = fputcsv($fp, $header);
    foreach ($data as $row) {
        foreach ($row as $key => $value) {
            if (is_array($value)) {
                $row[$key] = empty($value) ? null : json_encode($value);
            }
        }
        $fields = [];
        foreach ($header as $key) {
            $fields[$key] = isset($row[$key]) ? $row[$key] : null;
        }
        $len += fputcsv($fp, $fields);
    }
    fclose($fp);
    return $len;
}

echo json2csv($argv[1], $argv[2]);