HTML Guides for php
Learn how to identify and fix common HTML validation errors flagged by the W3C Validator — so your pages are standards-compliant and render correctly across every browser. Also check our Accessibility Guides.
XML processing instructions are a feature of XML, not HTML. They begin with <? and end with ?>, and are used in XML documents to carry instructions for applications processing the document. The most common example is the XML declaration: <?xml version="1.0" encoding="UTF-8"?>. While these are perfectly valid in XML, the HTML parser does not recognize them. When the parser encounters <?, it doesn’t know how to handle it and treats it as a bogus comment, which leads to unexpected behavior and this validation error.
This matters for several reasons. First, standards compliance — HTML5 has a clearly defined parsing algorithm, and processing instructions are not part of it. Second, browser behavior becomes unpredictable — different browsers may handle the unexpected <? content differently, potentially exposing raw code or breaking your layout. Third, if server-side code like PHP leaks into the output, it can expose sensitive logic or configuration details to end users.
There are three common causes of this error:
1. Inlining SVG files with their XML declaration. When you copy the contents of an .svg file and paste it directly into HTML, the file often starts with an XML declaration and possibly a <?xml-stylesheet?> processing instruction. These must be removed — only the <svg> element and its children should be included.
2. Unprocessed server-side code. Languages like PHP use <?php ... ?> (or the short tag <? ... ?>) syntax. If the server isn’t configured to process PHP files, or if a .html file contains PHP code without being routed through the PHP interpreter, the raw <?php tags end up in the HTML sent to the browser.
3. Copy-pasting XML content. Other XML-based formats (like RSS feeds, XHTML fragments, or MathML with XML declarations) may include processing instructions that aren’t valid in HTML.
How to Fix It
- For inline SVGs: Open the SVG file in a text editor and copy only the <svg>...</svg> element. Remove the <?xml ...?> declaration and any other processing instructions that precede the <svg> tag.
- For PHP or other server-side code: Ensure your server is properly configured to process the files. Check that the file extension matches what the server expects (e.g., .php for PHP files). Verify that the server-side language is installed and running.
- For other XML content: Strip out any <?...?> processing instructions before embedding the content in HTML.
Examples
❌ Inline SVG with XML declaration
<body>
<?xml version="1.0" encoding="UTF-8"?>
<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 100">
<circle cx="50" cy="50" r="40" fill="blue" />
</svg>
</body>The <?xml version="1.0" encoding="UTF-8"?> line triggers the error.
✅ Inline SVG without XML declaration
<body>
<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 100">
<circle cx="50" cy="50" r="40" fill="blue" />
</svg>
</body>❌ Unprocessed PHP code in HTML output
<body>
<h1>Welcome</h1>
<p><?php echo "Hello, World!"; ?></p>
</body>If the server doesn’t process the PHP, the raw <?php ... ?> ends up in the HTML and triggers this error.
✅ Properly processed output (or static HTML equivalent)
<body>
<h1>Welcome</h1>
<p>Hello, World!</p>
</body>❌ SVG with an XML stylesheet processing instruction
<body>
<?xml-stylesheet type="text/css" href="style.css"?>
<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 50 50">
<rect width="50" height="50" fill="red" />
</svg>
</body>✅ SVG without the processing instruction
<body>
<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 50 50">
<rect width="50" height="50" fill="red" />
</svg>
</body>In HTML, use standard <link> or <style> elements in the <head> for stylesheets instead of XML processing instructions.
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