Create a ZUGFeRD / Factur-X e-invoice in PHP

What you need

• An API key. Create one in the app under API keys and export it as FORMFEED_API_KEY. A test key (ff_test_…) is right for trying things out. See the quick start.
• PHP. You need PHP 8 with the curl and json extensions, which almost every installation has.
• The template. e-rechnung from the e-invoice example: copy it into your workspace from the example’s page, or push the folder with the CLI (formfeed templates push --publish). Any invoice template works the same way once its data has an _invoice block.
• The data. The example’s data.json next to the program: one _invoice block with seller, buyer, lines, the VAT breakdown and the totals, in the terms of the European standard EN 16931.
• A plan with e-invoices. Live e-invoices need the Starter plan or above. With a test key (ff_test_…) every plan makes test e-invoices with a test mark on the page, the Free plan up to 20 a day.

The program

The program reads data.json, posts it with the template’s name and post.einvoice, waits for the result when the API answers 202, stops with the API’s message when something is wrong, and saves the e-invoice.

<?php
$data = json_decode(file_get_contents('data.json'), true);
$body = [
    'template' => 'e-rechnung',
    'data' => $data,
    'post' => [
        'einvoice' => ['profile' => 'en16931'],
    ],
];

// one call with the key: the decoded answer (a render or a problem document) and its status code
function api(string $url, ?array $body = null): array {
    $curl = curl_init($url);
    curl_setopt_array($curl, [
        CURLOPT_RETURNTRANSFER => true,
        CURLOPT_HTTPHEADER => [
            'Authorization: Bearer ' . getenv('FORMFEED_API_KEY'),
            'Content-Type: application/json',
        ],
    ] + ($body === null ? [] : [CURLOPT_POST => true, CURLOPT_POSTFIELDS => json_encode($body)]));
    $answer = curl_exec($curl);
    // a connection that never got an answer: no status code to look at
    if ($answer === false) {
        fwrite(STDERR, curl_error($curl) . PHP_EOL);
        exit(1);
    }
    return [json_decode($answer, true), curl_getinfo($curl, CURLINFO_RESPONSE_CODE)];
}

[$render, $status] = api('https://api-eu.formfeed.dev/v1/renders', $body);

// not done within 60 seconds: the answer is 202 and the render goes on in the background;
// its result answers the same way until the render has finished
while ($status === 202) {
    sleep(2);
    [$render, $status] = api('https://api-eu.formfeed.dev/v1/renders/' . $render['id'] . '/result');
}

// every error is a problem document (RFC 9457) with a title and a detail
if ($status >= 400) {
    fwrite(STDERR, $render['title'] . ': ' . ($render['detail'] ?? '') . PHP_EOL);
    exit(1);
}

file_put_contents('e-invoice.pdf', file_get_contents($render['download_url']));
echo $render['id'], ' ', $render['page_count'], ' page(s), ', $render['units'], ' unit(s)', PHP_EOL;

Run it

It prints the render’s id, the page count and the units it cost (on a live key one more than the same PDF without the XML), and e-invoice.pdf lies next to the program. A PDF viewer shows the invoice, and its attachments show the factur-x.xml inside. With a test key the render is not metered, and every test e-invoice carries a test mark, on every plan.

export FORMFEED_API_KEY=ff_test_...
php render.php

What is particular about PHP

• Arrays become JSON. json_encode turns the nested array into the request body; json_decode(…, true) gives the sample data and the answer back as arrays.
• The download. file_get_contents on the download_url needs allow_url_fopen. Where that is off, fetch the URL with a second cURL handle; it is signed, so it needs no Authorization header.
• No SDK needed. There is no PHP package to install or keep up to date: the API is one POST with a JSON body.

What makes the PDF an e-invoice

• post.einvoice. turns the PDF into a PDF/A-3 that carries the invoice as factur-x.xml. The example template declares the same in its settings.json, so for this template the line is optional; for your own invoice template it is the one switch.
• The _invoice block. Seller, buyer, lines, VAT breakdown and totals, named as in the standard; units and VAT categories are codes (HUR, S). The template prints the page from the same block, so the page and the XML cannot say different things.
• The check. Before the file is delivered, the XML is held to the rules of EN 16931 and the PDF to PDF/A-3b. The report is in the render’s einvoice.validation, and a value the page does not show is listed in einvoice.display.missing.
• ZUGFeRD or Factur-X. One specification under two names, and the same file. flavour decides only which name and release the render reports: ZUGFeRD 2.5 or Factur-X 1.09.

When the data is wrong

A block that breaks a rule fails the request with 422 einvoice_data_invalid before anything is rendered or counted. The problem lists every finding with its place in the data and the rule it breaks (BR-CO-15, …), and the program prints its title and detail. A file that fails the check is never delivered, and never replaced by a plain PDF.

What stays with you

Formfeed creates and checks the file. Sending it to your customer, archiving it and the VAT treatment of the invoice stay with you. When an e-invoice is required, with the official sources, is on the e-invoicing page; the fields and the rules are in the e-invoice docs.

When the API says no

• One shape for every error. Errors are problem documents (RFC 9457) with type, title, status, detail, a stable code and the request_id to quote to support; the type URL leads to a page about that error. The list is in the error reference.
• The usual ones. 401 unauthorized for a missing or revoked key, 404 template_not_found for a wrong template name, 422 template_runtime_error when the template fails on the data, 429 rate_limited above the plan’s requests per second, 402 quota_exceeded when the Free plan’s units are used up (a paid plan bills extra units up to its hard cap).
• Retry safely. Send an Idempotency-Key header with the POST, and repeat the request with the same key when you do not know whether the first attempt arrived: for 24 hours the key returns the first answer instead of rendering and billing again. The answer is stored once the first attempt has been answered, so do not retry while it may still run; and problems below 500 are stored too, so a corrected request needs a new key.

The job around this program

This guide is one program. What the job needs around it, from the template to the limits and the price, is on the page Invoice PDF API.

More PHP guides

• Generate a PDF invoice from JSON in PHP
• Convert HTML to PDF in PHP
• Generate Open Graph images in PHP
• Generate PDFs from a CSV in PHP
• Merge PDFs and add a password in PHP
• Async PDF rendering with a verified webhook in PHP

FAQ

Which profile should I choose?

en16931, the default, carries every field of the European standard; basic is the smallest profile that is an invoice on its own. Any other profile is answered with einvoice_profile_unsupported.

Does Formfeed create XRechnung?

No. Formfeed writes ZUGFeRD / Factur-X: the XML inside a PDF/A-3. XRechnung and UBL are not available.

What does an e-invoice cost?

One unit on top of its render, so a one-page e-invoice costs two units. Live e-invoices need the Starter plan or above; on the Free plan, test renders make up to 20 a day. See pricing.

Can I have the XML as a file of its own?

xml: "both" in post.einvoice stores the XML beside the PDF as well, and the render links it as einvoice.xml_url. Inside the PDF it is there either way.

ZUGFeRD e-invoice in other languages

• Node.js (TypeScript)
• Python
• Go
• C# (.NET)
• Java
• Ruby
• curl