How to Fix PHP HTML-to-PDF Converters That Return Empty PDFs

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A “blank PDF” can mean several different failures: a zero-byte download, a valid PDF containing an empty page, a truncated or corrupt file, or an HTML/PHP error that your endpoint mislabeled as application/pdf. Identify which artifact you actually received before changing the template. Then inspect PHP and library errors, input size, output buffering, temporary directories, asset permissions, and renderer limitations in that order.

First, identify what “empty” means

Save the response instead of relying on the browser preview. Record the HTTP status, Content-Type, byte count, and first bytes of the file.

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curl -i "https://example.test/export.php?id=42" -o response.bin
wc -c response.bin
xxd -l 16 response.bin

A real PDF normally starts with the ASCII signature %PDF-. Use a PDF parser or desktop viewer to distinguish these cases:

• Zero bytes: the application returned nothing, terminated early, or failed before writing the document.
• Valid PDF with a blank page: the renderer ran, but received no usable content or discarded it during layout.
• Corrupt or truncated PDF: output was interrupted, mixed with other bytes, or the process ran out of memory or storage.
• HTML/PHP error saved as a PDF: warnings, an exception page, or a login response was sent with the wrong content type.

Check the PHP-FPM/Apache error log and the application log at the same timestamp. Do not debug only the browser tab: mPDF’s blank-screen guidance notes that script errors and memory exhaustion can produce blank browser output.

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Use a minimal PHP reproduction

Remove your framework controller, database query, images, and large stylesheet temporarily. Render a known-good document and write it to a file rather than streaming it.

<?php
require __DIR__ . '/vendor/autoload.php';

use MpdfMpdf;
use MpdfConfigConfigVariables;
use MpdfConfigFontVariables;

$temporaryDirectory = __DIR__ . '/var/mpdf-temp';
if (!is_dir($temporaryDirectory) && !mkdir($temporaryDirectory, 0750, true)) {
    throw new RuntimeException('Cannot create mPDF temporary directory');
}

$mpdf = new Mpdf(['tempDir' => $temporaryDirectory]);
$mpdf->WriteHTML('<h1>PDF smoke test</h1><p>Renderer is running.</p>');
$mpdf->Output(__DIR__ . '/var/smoke-test.pdf', MpdfOutputDestination::FILE);

clearstatcache(true, __DIR__ . '/var/smoke-test.pdf');
$size = filesize(__DIR__ . '/var/smoke-test.pdf');
if ($size === false || $size < 100) {
    throw new RuntimeException('PDF was not written or is unexpectedly small');
}

If this fails, the problem is environmental or library-level, not your business template. If it succeeds, add your real HTML in small increments until the failure returns.

Surface exceptions and PHP errors

In development, enable error reporting and catch the renderer’s exception. In production, log details while returning a generic error to the client; never emit a stack trace into a PDF response.

<?php
ini_set('display_errors', '0');
ini_set('log_errors', '1');
error_reporting(E_ALL);

try {
    $mpdf = new MpdfMpdf(['tempDir' => __DIR__ . '/var/mpdf-temp']);
    $mpdf->WriteHTML($html);
    $bytes = $mpdf->Output('', MpdfOutputDestination::STRING_RETURN);

    if (strncmp($bytes, '%PDF-', 5) !== 0) {
        throw new RuntimeException('Renderer did not return a PDF signature');
    }

    header('Content-Type: application/pdf');
    header('Content-Disposition: inline; filename="document.pdf"');
    header('Content-Length: ' . strlen($bytes));
    echo $bytes;
} catch (MpdfMpdfException $e) {
    error_log('PDF generation failed: ' . $e->getMessage());
    http_response_code(500);
    header('Content-Type: application/json');
    echo json_encode(['error' => 'PDF generation failed']);
}

Also inspect web-server logs, PHP-FPM logs, container logs, and the process memory limit. A blank response after a fatal error often means the error was logged but never shown to the caller.

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Check for output sent before PDF generation

Any notice, warning, debug echo, whitespace before <?php, or included file that prints text can corrupt a streamed PDF. mPDF documents this failure mode on its error-messages page.

• Remove closing PHP tags from PHP-only files.
• Search included files for echo, var_dump, accidental HTML, and UTF-8 byte-order marks.
• Do not send framework middleware output, debug toolbars, or a session warning on the PDF route.
• Clear an intentionally used output buffer only after understanding why it exists.

PHP’s output-buffering manual explains that buffered output is held until flushed. Buffering can be useful, but blindly calling ob_end_clean() may hide a real diagnostic or break middleware. During debugging, log the buffer length and contents rather than silently discarding them.

$level = ob_get_level();
if ($level > 0) {
    error_log('Output buffers before PDF: ' . $level);
}

Investigate mPDF’s large-input limit

If a short document works but a long invoice, report, or CSS-heavy page becomes blank or loses sections, test the size of the string passed to WriteHTML(). The mPDF known-issues documentation explains that oversized input can exceed PHP’s PCRE processing limit; a regular-expression operation may return null, leaving blank pages or missing content.

Split HTML and CSS into chunks

$mpdf->WriteHTML($baseCss, MpdfHTMLParserMode::HEADER_CSS);
foreach (array_chunk($sections, 1) as $section) {
    $mpdf->WriteHTML($section[0], MpdfHTMLParserMode::HTML_BODY);
}

Chunk at logical boundaries such as a page, table, or repeated report section. Avoid splitting in the middle of a tag. Reproduce with a minimal document, then add one section at a time so you can identify the first input that triggers the limit.

Review the PCRE setting carefully

Check the effective value in the same SAPI that runs the request:

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<?php
var_dump(PHP_SAPI, ini_get('pcre.backtrack_limit'), ini_get('memory_limit'));

If your hosting policy permits it, raise pcre.backtrack_limit for the worker and retest. A higher limit consumes more CPU and memory; splitting the input is usually safer for repeatable production jobs. Do not assume a setting changed in CLI also applies to PHP-FPM.

Verify temporary storage and permissions

Renderers need scratch space for fonts, images, and intermediate data. mPDF’s temporary-file guidance requires a writable directory and warns against making it world-writable with 777.

1. Create a directory outside the public web root, such as /var/app/mpdf-temp.
2. Give ownership to the actual PHP-FPM, Apache, or CLI user that runs the job.
3. Use restrictive permissions (for example, owner read/write/execute and no public access).
4. Confirm free disk space and inode availability.
5. Run a smoke test through the same web endpoint, not only from your shell.

In containers, remember that a writable path may be ephemeral. Mount persistent or adequately sized temporary storage when jobs can run concurrently.

Check images, stylesheets, fonts, and remote-resource rules

A PDF can be structurally valid yet appear empty when its visible content depends on blocked assets. Test with plain text first, then add one asset at a time.

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• Use absolute, correctly encoded local paths and confirm the PHP worker can read them.
• Check HTTPS certificate validation, redirects, authentication, and DNS from the server—not your laptop.
• Confirm that CSS URLs, web fonts, and images are allowed by the renderer’s security settings.
• Log HTTP status and byte size for each downloaded asset; a 200 response containing an HTML login page is not an image.

Dompdf’s usage documentation covers its temporary-directory, remote-resource, and logging options. The current TCPDF HTML/CSS renderer documents local-path and remote-host allowlists at tcpdf.org/docs/html-css/; an unallowed asset may be refused without an exception. Treat those settings as security boundaries rather than enabling unrestricted filesystem or network access.

Compare your HTML/CSS with the renderer’s capabilities

PHP PDF libraries are not full browsers. If text appears but grid, flexbox, sticky elements, filters, or complex responsive rules disappear, reduce the template to the renderer’s supported subset. mPDF describes its CSS support as limited compared with modern browser rendering and points to headless Chrome when reproducing a modern web page is the priority; see the mPDF project README.

Choose the rendering approach against requirements

There is no universal best library. Fix PHP errors, permissions, and invalid responses before changing renderers; a browser engine will not repair a broken endpoint.

A repeatable diagnostic checklist

1. Capture status, content type, byte count, and the first bytes of the response.
2. Write a minimal PDF to disk and verify it independently of HTTP streaming.
3. Enable logging, catch library exceptions, and inspect PHP-FPM/web-server logs.
4. Remove all output before generation and audit buffering.
5. Compare small and large inputs; inspect PCRE and memory limits for mPDF.
6. Verify the temp directory, ownership, permissions, disk space, and runtime user.
7. Test text, CSS, local assets, then remote assets separately.
8. Reduce unsupported CSS or evaluate a headless-browser approach when browser fidelity is essential.
9. Restore the production template incrementally and keep the smallest failing fixture as a regression test.

Or skip the browser setup

If your actual goal is a screenshot or PDF of a web page rather than server-side PHP template rendering, ScreenshotNeo provides a single website-screenshot API request. It accepts consent banners like a visitor and removes more than 60 known consent platforms, newsletter popups, and chat widgets before capture; bot checks/CAPTCHAs, blank pages, timeouts, failed loads, and cache hits are not billed, and response headers identify the page verdict and billing result.

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cURL

curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webp

Python

import requests
r = requests.get("https://api.screenshotneo.com/v1/shot", params={"access_key": "YOUR_API_KEY", "url": "https://stripe.com"}, timeout=90)
r.raise_for_status()
open("shot.webp", "wb").write(r.content)

Node.js

const q = new URLSearchParams({ access_key: 'YOUR_API_KEY', url: 'https://stripe.com' });
const res = await fetch(`https://api.screenshotneo.com/v1/shot?${q}`);
if (!res.ok) throw new Error(`HTTP ${res.status}`);
const fs = await import('node:fs/promises');
await fs.writeFile('shot.webp', Buffer.from(await res.arrayBuffer()));

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Common symptoms and targeted fixes

FAQ

Why is the browser blank but the downloaded file valid?

Browser viewers can fail to display a valid file because of caching, an incomplete response, or viewer limitations. Save the response and validate its PDF signature and page structure with an independent viewer or parser.

Should I enable PHP error display on a production PDF endpoint?

No. Log errors server-side and return a correctly typed error response. Displayed warnings become bytes in the PDF stream and can expose sensitive data.

Can increasing memory or PCRE limits guarantee a fix?

No. Those changes help only when the corresponding limit is the failure boundary. Confirm with a minimal reproduction and logs, then prefer smaller chunks and simpler input where practical.

When is a headless browser the wrong choice?

It may be unsuitable when the host cannot install or sandbox a browser, when startup cost is unacceptable, or when a simpler PHP renderer already meets your pagination and CSS needs.

Frequently Asked Questions

How do I tell whether a PDF is empty or actually an HTML error?

Check the HTTP status, Content-Type, byte count, and first bytes. A PDF begins with %PDF-; an HTML error usually begins with markup or an error message.

What is the safest way to debug output corruption?

Write the PDF to a file first, log exceptions, and audit warnings, debug output, whitespace, and output buffers before streaming it.

Does ScreenshotNeo replace a PHP HTML-to-PDF library?

It is an alternative for capturing an existing web page as an image or PDF through an API; it does not fix a PHP template renderer that is failing inside your application.

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