Fix Invalid XML Sitemap Error in WordPress (Quick Guide)

1️⃣ Introduction

An XML sitemap is very important for SEO. It helps Google and Bing discover and index your website faster.

But sometimes, WordPress sites face this error:
“XML declaration allowed only at the start of the document.”

When this happens:

• Google Search Console flags your sitemap.

• SEO tools like Semrush or Ahrefs report syntax issues.

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• Your indexing speed slows down.

👉 This guide is based on a real fix from SinhMMO. You can solve it yourself in under 15 minutes.

2️⃣ Root Causes

This sitemap error usually comes from three common problems:

a) Extra spaces in core files

• wp-config.php or functions.php may contain blank lines before the <?php tag.

• Sometimes developers leave ?> at the end of files.

• Even a single space can break your XML.

b) Wrong snippet or plugin output

• PHP snippets added via WPCode or Code Snippets may use echo outside of hooks.

• Plugins like Ad Inserter, GTranslate, or Shortcodes Ultimate sometimes output headers incorrectly.

• This creates unwanted content before the XML starts.

c) SEO plugin conflicts

• Yoast SEO, RankMath, or All in One SEO can conflict with other plugins.

• Custom or nulled themes may also output duplicate XML.

3️⃣ How to Detect the Error

Here are simple ways to confirm the problem:

• Check your sitemap URL: Visit https://yourdomain.com/sitemap_index.xml. If you see an error on line 2, the issue exists.

• Google Search Console: Re-submit your sitemap. If it says “Couldn’t fetch”, there’s a syntax error.

• SEO tools: Run a site audit with Semrush or Ahrefs. Look for sitemap errors.

• View source: Right-click ➜ View page source. If you see spaces before <?xml, that’s the culprit.

4️⃣ Step-by-Step Fix

Follow these steps to remove the error permanently:

Step 1: Fix wp-config.php

• Open the file.
• Remove any spaces before <?php.
• Delete the closing ?> at the end.
• Save and clear your cache.

Step 2: Fix functions.php

• Open your theme’s functions.php.
• At the top, add:

<?php
ob_start();

• Make sure no echo is outside of hooks.

Step 3: Create a whitespace fix file

• On your server, create a new file, e.g., whitespacefix.php.
• Paste this code:

<?php 
function ___wejns_wp_whitespace_fix($input) { 
  $allowed = false; 
  $found = false; 
  foreach (headers_list() as $header) { 
    if (preg_match("/^content-type:\\s+(text\\/|application\\/((xhtml|atom|rss)\\+xml|xml))/i", $header)) { 
      $allowed = true; 
    } 
    if (preg_match("/^content-type:\\s+/i", $header)) { 
      $found = true; 
    } 
  } 
  if ($allowed || !$found) { 
    return preg_replace("/\\A\\s*/m", "", $input); 
  } else { 
    return $input; 
  } 
} 
ob_start("___wejns_wp_whitespace_fix"); 
?>

Step 4: Include the file in index.php

• Open index.php.
• Add this line right after <?php:

include('whitespacefix.php');

• Visit https://yourdomain.com/whitespacefix.php.
• Then check your sitemap again.

Step 5: Re-submit to Google

• Go to Search Console.
• Remove the old sitemap.
• Submit your new one.
• Wait for Google to process it. It should now show Success.

5️⃣ Results & Key Takeaways

After applying the fix:

• Your sitemap becomes valid.
• SEO tools no longer show errors.
• New posts index faster.
• Your site runs smoother without header conflicts.

📌 Pro Tips

• Always back up your site before editing.
• Use only trusted plugins.
• Add a clean robots.txt:

User-agent: *
Allow: /
Sitemap: https://yourdomain.com/sitemap_index.xml

• Audit your sitemap regularly with Search Console.

✅ Conclusion

This is the exact method SinhMMO used to fix a real-life XML sitemap error.

👉 If you face the same issue, follow these steps.
And if you still get stuck, drop a comment on SinhMMO.com — we’re always ready to help.

🎉 Happy fixing! May your site get indexed lightning-fast.