Use a worker that renders each record into print-focused HTML, converts it with an isolated PHP PDF renderer, enforces page parity at every record boundary, and validates the finished file before delivery. For duplex jobs, the difficult part is not producing a PDF; it is guaranteeing that every front and back lands on the intended sheet side. The workflow below covers renderer choice, HTML/CSS constraints, PHP code, parity rules, validation, scaling, and recovery from common failures.
Choose the renderer for your template
Dompdf, mPDF, and TCPDF (including the newer tc-lib-pdf line) can generate PDFs from HTML, but they are not browser engines. Design the template for the renderer you select rather than assuming that browser CSS will work unchanged.
| Renderer | What it handles well | Important constraints for duplex batches |
|---|---|---|
| Dompdf | PHP HTML-to-PDF conversion with a mostly CSS 2.1 layout engine and selected CSS3 features such as @import, @media, and @page. |
The project warns that one Dompdf instance should not render more than one HTML document, because parsing and rendering artifacts can persist into later renders. Create a fresh instance for each document or isolated batch unit. |
| mPDF | UTF-8 HTML, headers and footers, page numbering, tables of contents, barcodes, and documented conditional page breaks for double-sided output. | Its manual describes the library as dated; templates often need mPDF-specific CSS and markup rather than modern browser layout. |
| TCPDF / tc-lib-pdf | A supported HTML/CSS subset without a browser engine. addHTMLCell() can perform automatic page and region breaks, while getHTMLCell() returns operators for caller-controlled placement. Paged-media controls include page-break rules, orphans, widows, and print media. |
You must stay inside the documented HTML/CSS subset and take responsibility for placement when exact front/back registration matters. |
Compare candidates on CSS fidelity, @page and break-rule support, Unicode and font embedding, parity controls, memory behavior, deployment dependencies, and how easily you can inspect record boundaries. Do not use a throughput number from another project: measure your PHP version, template, fonts, images, and hardware together.
Design the duplex document before writing PHP
Define the logical unit
Decide whether one record is one page, a front/back pair, or a variable-length section. Give each logical side a self-contained wrapper and an explicit boundary. If a record must always occupy exactly two pages, the parity calculation is simple: every new record starts on the same side. Variable-length records require conditional parity breaks.
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Use print CSS that the engine supports
- Set paper size and margins with
@page; leave a safe margin for the printer’s non-printable area and any bleed. - Use explicit page-break markers at record boundaries. Do not rely on flexbox, grid, JavaScript, or generated content unless your selected engine documents support for them.
- Reserve space for headers, footers, barcodes, and registration marks in the layout, not by letting content push them onto another page.
- Embed the exact font files required for Unicode text and consistent metrics. A fallback font can change line wrapping and therefore page parity.
- Prepare images at the resolution needed at final paper size; oversized source images increase memory use without improving output.
Make front/back geometry intentional
For short-edge or long-edge binding, the printer may mirror the back side differently. Place marks and artwork according to the binding edge, and test a physical two-page sample. A PDF can have correct page numbers while still producing an upside-down or laterally misregistered back.
PHP batch architecture
- Load and validate rows. Reject missing identifiers, malformed URLs, invalid dates, and values outside the template’s limits before starting rendering.
- Render one record (or a bounded chunk). Escape all data and use local, deterministic assets. Never let an arbitrary row inject HTML or CSS.
- Create a fresh renderer. Isolate an instance per document or bounded batch unit. This is mandatory for Dompdf and a prudent memory boundary for every renderer.
- Apply fixed page geometry and breaks. Keep the same paper size, margins, fonts, and break markers throughout a batch.
- Track parity. Before a record that must begin on an odd or even page, issue a conditional break. If the current page already has the requested parity, no blank page is added.
- Write a temporary PDF. Inspect it before publication, then move it atomically to the final path.
- Log reproducibility data. Store the input ID, renderer and PHP versions, template revision, page count, and validation result.
Complete mPDF example with record boundaries
The following example assumes each record has a front and a back section. It uses mPDF’s conditional page-break type: O requests an odd-page start and E requests an even-page start. Confirm the exact method signature against the mPDF version installed in your application.
<?php
declare(strict_types=1);
require __DIR__ . '/vendor/autoload.php';
use MpdfMpdf;
use MpdfOutputDestination;
$rows = [
['id' => 'A-100', 'name' => 'Ada Lovelace', 'address' => '1 Example Street'],
['id' => 'B-200', 'name' => 'Grace Hopper', 'address' => '2 Example Street'],
];
function e(string $value): string {
return htmlspecialchars($value, ENT_QUOTES | ENT_SUBSTITUTE, 'UTF-8');
}
function front(array $row): string {
return '<section class="side front">'
. '<h1>Customer card</h1>'
. '<p class="id">Record: ' . e($row['id']) . '</p>'
. '<p>' . e($row['name']) . '<br>' . e($row['address']) . '</p>'
. '</section>';
}
function back(array $row): string {
return '<section class="side back">'
. '<h2>Instructions</h2>'
. '<p>Keep this side aligned with the front when duplex printing.</p>'
. '</section>';
}
$css = <<<'CSS'
@page { size: A4; margin: 14mm 14mm 16mm 14mm; }
body { font-family: sans-serif; font-size: 11pt; }
.side { page-break-after: always; min-height: 250mm; position: relative; }
.side:last-child { page-break-after: auto; }
.id { font-size: 9pt; }
CSS;
$mpdf = new Mpdf([
'format' => 'A4',
'margin_left' => 14,
'margin_right' => 14,
'margin_top' => 14,
'margin_bottom' => 16,
]);
$mpdf->WriteHTML($css, MpdfHTMLParserMode::HEADER_CSS);
$first = true;
foreach ($rows as $row) {
if (!$first) {
// Start the next record on an odd page; mPDF adds a blank page only when needed.
$mpdf->AddPage('', '', 'O');
}
$mpdf->WriteHTML(front($row), MpdfHTMLParserMode::HTML_BODY);
$mpdf->WriteHTML('<pagebreak />', MpdfHTMLParserMode::HTML_BODY);
$mpdf->WriteHTML(back($row), MpdfHTMLParserMode::HTML_BODY);
$first = false;
}
$tmp = tempnam(sys_get_temp_dir(), 'cards-');
$mpdf->Output($tmp, Destination::FILE);
$final = __DIR__ . '/out/cards.pdf';
if (!is_dir(dirname($final))) {
mkdir(dirname($final), 0775, true);
}
rename($tmp, $final); // atomic on the same filesystem
echo "Wrote {$final}n";
For a record that must begin on an even page, use AddPage('', '', 'E'). If your installed mPDF release exposes a different argument order, use its AddPage reference and retain the same rule: request the desired parity and let the renderer add a page only when the current page does not match.
Dompdf and TCPDF implementation notes
Dompdf
Build one complete HTML document per isolated Dompdf instance. Set paper and orientation explicitly, load only approved local assets, call render(), and save the output. Do not reuse the object for the next document; persisted parsing and rendering artifacts can affect later renders. Dompdf’s CSS support is strongest when the template stays close to CSS 2.1 with its documented additions.
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TCPDF or tc-lib-pdf
Use addHTMLCell() when the library should manage automatic page and region breaks. Use getHTMLCell() when your application must place the returned operators at an exact position. Keep HTML simple and use the documented paged-media controls for print output. For duplex jobs, explicit page starts are safer than allowing a long cell to flow across a record boundary.
Validation before releasing a batch
- Open the PDF and verify the expected page count.
- Check that every record starts on its intended odd or even page.
- Confirm no record crosses a forced boundary unexpectedly.
- Inspect front/back artwork for the printer’s binding-edge orientation.
- Verify fonts, Unicode text, images, and barcodes at 100% print scale.
- Ensure every blank parity page is intentional and represented in internal logs or metadata.
- Run a small deterministic fixture after changing PHP, the renderer, fonts, images, or the template.
Validation can be automated with the PDF parser or command-line inspection tools already approved in your deployment. Treat a page-count or boundary mismatch as a failed job, not as a warning.
Scaling, memory, and reliability
Use bounded work units
Very large HTML documents consume memory in both parsing and layout. Process a bounded number of records per worker, write each unit to a temporary file, and merge only if your PDF toolchain supports a tested merge step. Isolated units also make retries cheaper and prevent one malformed row from invalidating an entire run.
Make jobs repeatable
Pin the renderer and PHP versions, template revision, font files, and image assets. Keep inputs ordered by a stable key. Record a checksum or revision for each template so a regenerated batch can be compared with the original.
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Handle failures safely
Use a queue with retries for transient asset or filesystem failures, but do not retry deterministic validation failures indefinitely. Keep the temporary file private, verify it, then atomically rename it. Never replace a previously released PDF with a partially rendered file.
Common failures and fixes
Front and back drift to the wrong side
The document is missing an explicit boundary or the parity rule is applied after content has already been written. Add the conditional break before the next record and log the page number at that point.
A blank page appears unexpectedly
A conditional break is doing its job, or an extra CSS break is present. Inspect both the renderer call and the template’s page-break-* rules; keep one authoritative boundary mechanism.
Content is clipped or overlaps the footer
Margins, header/footer reservations, or a fixed-height container exceed the printable area. Reduce content, increase the margin, or move the element into the normal flow. Do not solve it with a browser-only CSS rule the renderer does not support.
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Unicode characters become boxes
The required font is not embedded or the renderer cannot locate it. Configure and embed the exact font files, verify the font license for server use, and rerun the deterministic fixture.
Images or barcodes disappear
The asset path is unavailable to the worker, remote retrieval is blocked, or the format is unsupported. Prefer local, deterministic assets and check the renderer’s supported image formats and permissions.
Later documents change after rendering earlier ones
This is a classic instance-reuse problem, especially with Dompdf. Construct a new renderer for each document or isolated batch unit.
The worker runs out of memory
Reduce the batch size, avoid embedding unnecessarily large images, and recycle the renderer between units. Measure the target workload; no universal throughput or memory figure applies across templates and environments.
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cURL
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r.raise_for_status()
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Should each record be rendered as its own PDF?
Use separate PDFs when independent delivery or retrying individual records matters; use one batch PDF when a print run must remain a single ordered file. In either case, preserve the same parity and validation rules.
How do I test duplex alignment without wasting a full print run?
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The Bottom Line
Reliable duplex PDFs come from deterministic templates, isolated renderer instances, explicit parity breaks, and automated boundary validation. Choose the engine whose supported CSS matches your template, then treat every unvalidated page as a failed batch.
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