PHP XMLWriter

What Is XMLWriter?

In simple terms: XMLWriter creates XML documents step by step: "start an element", "add an attribute", "write some text", "close the element". It handles escaping and indentation for you and can write directly to a file or the browser while generating — so even a sitemap with a million URLs can be produced without building it in memory first.

Technically: The XMLWriter class is a forward-only, streaming XML generator built on libxml. Output goes to memory (openMemory()) or to a URI/stream (openUri()), and can be flushed in portions with flush(). It is the writing counterpart of XMLReader: fast and memory-efficient, but it cannot go back and modify what was already written.

Choosing an XML Generation Tool

Writing a Document

<?php
$writer = new XMLWriter();
$writer->openMemory();
$writer->setIndent(true);
$writer->setIndentString('  ');

$writer->startDocument('1.0', 'UTF-8');

$writer->startElement('order');
$writer->writeAttribute('id', '501');

$writer->startElement('item');
$writer->writeAttribute('sku', 'PEN-01');
$writer->writeElement('name', 'Gel Pen');
$writer->writeElement('quantity', '10');
$writer->endElement();   // </item>

$writer->writeElement('note', 'Ring the bell & wait');

$writer->endElement();   // </order>
$writer->endDocument();

echo $writer->outputMemory();

Output

<?xml version="1.0" encoding="UTF-8"?>
<order id="501">
  <item sku="PEN-01">
    <name>Gel Pen</name>
    <quantity>10</quantity>
  </item>
  <note>Ring the bell & wait</note>
</order>

How It Works

Text and attribute values are escaped automatically: & became &. Values must be strings; convert numbers with (string) or string interpolation.

Practical Example: Generating a Sitemap

Search engines read sitemaps to discover the pages of a website. A sitemap is an XML document in the http://www.sitemaps.org/schemas/sitemap/0.9 namespace:

<?php
function writeSitemap(XMLWriter $writer, iterable $pages): void
{
    $writer->startDocument('1.0', 'UTF-8');
    $writer->startElement('urlset');
    $writer->writeAttribute('xmlns', 'http://www.sitemaps.org/schemas/sitemap/0.9');

    $count = 0;
    foreach ($pages as $page) {
        $writer->startElement('url');
        $writer->writeElement('loc', $page['url']);
        $writer->writeElement('lastmod', $page['updated']->format('Y-m-d'));
        $writer->endElement();

        if (++$count % 1000 === 0) {
            $writer->flush();   // write buffered output to the destination
        }
    }

    $writer->endElement();
    $writer->endDocument();
    $writer->flush();
}

$pages = [
    ['url' => 'https://gocourse.in/products/gel-pen', 'updated' => new DateTimeImmutable('2026-10-15')],
    ['url' => 'https://gocourse.in/products/desk-lamp', 'updated' => new DateTimeImmutable('2026-10-12')],
];

$writer = new XMLWriter();
$writer->openUri('php://output');
$writer->setIndent(true);
$writer->setIndentString('  ');

writeSitemap($writer, $pages);

Output

<?xml version="1.0" encoding="UTF-8"?>
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
  <url>
    <loc>https://gocourse.in/products/gel-pen</loc>
    <lastmod>2026-10-15</lastmod>
  </url>
  <url>
    <loc>https://gocourse.in/products/desk-lamp</loc>
    <lastmod>2026-10-12</lastmod>
  </url>
</urlset>

Step-by-Step Explanation

1. openUri('php://output') sends the XML directly to the output (the browser, or the terminal in the command line). For a file, use openUri('/path/to/sitemap.xml').
2. The default namespace is declared with a normal xmlns attribute on the root element.
3. $pages is iterable, so it can be a generator that fetches rows from the database one at a time (see PHP Generators). Combined with flush() every 1,000 URLs, memory use stays constant for any number of pages.
4. A final flush() writes whatever remains in the buffer.

When serving the sitemap from a PHP script, send header('Content-Type: application/xml; charset=utf-8'); before writing. (A single sitemap file may contain at most 50,000 URLs; larger sites use a sitemap index.)

CDATA, Comments, and Namespaces

<?php
$writer = new XMLWriter();
$writer->openMemory();
$writer->setIndent(true);
$writer->setIndentString('  ');

$writer->startDocument('1.0', 'UTF-8');
$writer->writeComment(' Generated by GoCourse ');

$writer->startElementNs('g', 'feed', 'http://example.com/ns/shop');
$writer->startElement('description');
$writer->writeCdata('<p>Bold & bright <b>LED</b> lamp</p>');
$writer->endElement();
$writer->endElement();

$writer->endDocument();
echo $writer->outputMemory();

Output

<?xml version="1.0" encoding="UTF-8"?>
<!-- Generated by GoCourse -->
<g:feed xmlns:g="http://example.com/ns/shop">
  <description><![CDATA[<p>Bold & bright <b>LED</b> lamp</p>]]></description>
</g:feed>

• writeCdata() writes a CDATA section: content that is taken literally, without escaping — useful for embedding HTML snippets that the receiving system expects in that form. CDATA cannot contain the sequence ]]>.
• writeComment() adds an XML comment.
• startElementNs($prefix, $name, $uri) creates a namespaced element and declares the namespace.

PHP 8.4: Static Constructors

PHP 8.4 added clearer factory methods:

$writer = XMLWriter::toMemory();
$writer = XMLWriter::toUri('/var/www/public/sitemap.xml');
$writer = XMLWriter::toStream($resource);

They replace new XMLWriter() followed by openMemory() / openUri().

Common Mistakes

Writing Attributes After Content

Effect: Attributes are silently not written (or the call fails) once text or child elements have been added.

Fix: Call writeAttribute() immediately after startElement().

Unbalanced startElement() / endElement()

Effect: Elements are closed at the wrong level.

Fix: Keep start and end calls paired, or wrap sections in small functions; endDocument() closes any remaining elements.

Passing Non-String Values

Effect: TypeError in strict mode for integers or floats.

Fix: Convert values to strings explicitly.

Building XML With String Concatenation

Effect: Unescaped & or < produce broken documents.

Fix: Use XMLWriter, which escapes automatically.

Best Practices

• Use XMLWriter for generating large XML documents and streaming output.
• Enable indentation only when humans read the output; omit it for smaller files.
• Feed data from generators and flush() periodically for constant memory use.
• Send the correct Content-Type header when serving XML over HTTP.
• Use DOM instead when the document must be modified after creation.

Official reference: https://www.php.net/manual/en/book.xmlwriter.php