从零开发Sitemap插件遇XML声明错误，求技术支持

Let's break down why you're seeing that XML error and fix it step by step:

The Root Cause #

The error means there's hidden whitespace, a Byte Order Mark (BOM), or unintended output before the <?xml declaration in your generated sitemap.xml file. XML standards require the declaration to be the very first thing in the document—any leading characters break validity.

Fix 1: Properly Initialize the $sitemap Variable #

Your code starts appending to $sitemap with $sitemap .= ... without initializing it first. If PHP has any existing output (even invisible whitespace), this gets prepended to your XML. Add an explicit initialization at the top of your function:

function eg_create_sitemap() {
    $sitemap = ''; // Add this line to start with an empty string
    // Rest of your code...
}

Fix 2: Eliminate Unintended Output from Your Plugin File #

• Ensure your index.php file starts with <?php without any leading spaces, newlines, or BOM characters. Many editors add a BOM when saving UTF-8 files—switch to saving as "UTF-8 without BOM".
• Remove the closing ?> tag from your PHP file if it exists. PHP doesn't require it, and any whitespace after it will be output to the file, breaking your XML structure.

Fix 3: Clean Up GMT Offset Logic (More Reliable) #

Your current GMT offset code can produce invalid formats. Replace it with a simpler approach using WordPress's built-in timezone functions:

// Replace your existing $tempo logic with this
$timezone = wp_timezone();
$offset = $timezone->getOffset(new DateTime());
$tempo = sprintf('%+03d:00', $offset / 3600);

Fix 4: Correct Stylesheet Filename & XML Case Sensitivity #

You named your stylesheet sitemap-style.xsl but your code references sitemap.xsl—that's a typo! Also, XML is case-sensitive: valid changefreq values are lowercase (e.g., weekly, not Weekly).

Modified Working Code (All Fixes Applied) #

Here's your updated function with critical fixes:

add_action("publish_post", "eg_create_sitemap");
add_action("publish_page", "eg_create_sitemap");
add_action( "save_post", "eg_create_sitemap" );

function eg_create_sitemap() {
    $sitemap = ''; // Initialize empty string
    
    // Fixed GMT offset calculation
    $timezone = wp_timezone();
    $offset = $timezone->getOffset(new DateTime());
    $tempo = sprintf('%+03d:00', $offset / 3600);

    $postsForSitemap = get_posts( array(
        'numberposts' => -1,
        'orderby' => 'modified',
        'post_type' => array( 'post', 'page' ),
        'order' => 'DESC'
    ) );

    $sitemap .= '<?xml version="1.0" encoding="UTF-8"?>' . '<?xml-stylesheet type="text/xsl" href="' . esc_url( home_url( '/' ) ) . 'sitemap-style.xsl"?>'; // Fixed stylesheet filename
    $sitemap .= "\n" . '<urlset xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.sitemaps.org/schemas/sitemap/0.9 http://www.sitemaps.org/schemas/sitemap/0.9/sitemap.xsd" xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">' . "\n";
    $sitemap .= "\t" . '<url>' . "\n" . "\t\t" . '<loc>' . esc_url( home_url( '/' ) ) . '</loc>' . "\n\t\t" . '<lastmod>' . date( "Y-m-d\TH:i:s", current_time( 'timestamp', 0 ) ) . $tempo . '</lastmod>' . "\n\t\t" . '<changefreq>daily</changefreq>' . "\n\t\t" . '<priority>1.0</priority>' . "\n\t" . '</url>' . "\n";

    foreach( $postsForSitemap as $post ) {
        setup_postdata( $post);
        $postdate = explode( " ", $post->post_modified );
        $sitemap .= "\t" . '<url>' . "\n" . "\t\t" . '<loc>' . esc_url(get_permalink( $post->ID )) . '</loc>' . "\n\t\t" . '<lastmod>' . $postdate[0] . 'T' . $postdate[1] . $tempo . '</lastmod>' . "\n\t\t" . '<changefreq>weekly</changefreq>' . "\n\t\t" . '<priority>0.5</priority>' . "\n\t" . '</url>' . "\n"; // Fixed changefreq to lowercase
    }

    $sitemap .= '</urlset>';

    $fp = fopen( ABSPATH . "sitemap.xml", 'w' );
    fwrite( $fp, $sitemap );
    fclose( $fp );

    wp_reset_postdata(); // Reset postdata to avoid breaking other queries
}

Final Checks #

After applying these fixes:

1. Save a post or page to trigger the sitemap regeneration.
2. Open sitemap.xml in a text editor—confirm there's nothing before the <?xml declaration.
3. Visit https://mydomain/sitemap.xml; the XML error should be resolved, and your styled sitemap should load correctly.

内容的提问来源于stack exchange，提问作者Creative Tim