WordPress UTF-8 Text Handling

WordPress 6.9 modernized UTF-8 handling. New code should prefer the explicit UTF-8 helpers instead of older heuristics and hand-written byte regexes.

This skill is for server-side plugin text processing. It is not about block editor text controls.

When to use this skill

Trigger when ANY of the following is true:

• Code uses seems_utf8(), mb_check_encoding(), iconv(), utf8_encode(), utf8_decode(), or byte-level regexes.
• A plugin imports or exports CSV, XML, feeds, JSON, email, AI prompt data, logs, filenames, or third-party API text.
• The task mentions invalid UTF-8, mojibake, replacement character �, XML generation, REST encoding errors, or noncharacters.

The core helpers

seems_utf8() is deprecated in WP 6.9. Use wp_is_valid_utf8() for validation.

Validate, scrub, or reject

Pick behavior based on the boundary:

Replacing invalid bytes is lossy. Once U+FFFD is inserted, you cannot know what the original byte sequence was.

Examples

Reject invalid imported text:

$name = (string) ( $row['name'] ?? '' );

if ( ! wp_is_valid_utf8( $name ) ) {
    return new WP_Error(
        'myplugin_invalid_utf8',
        __( 'The imported name contains invalid UTF-8 bytes.', 'myplugin' )
    );
}

Scrub before XML output:

$title = wp_scrub_utf8( (string) $title );

$xml .= '<title>' . esc_xml( $title ) . '</title>';

Preserve raw bytes in storage, scrub for display:

update_post_meta( $post_id, '_myplugin_raw_payload', $payload );

$safe_for_screen = wp_scrub_utf8( $payload );
echo esc_html( $safe_for_screen );

wp_check_invalid_utf8()

wp_check_invalid_utf8( $text, false ) returns an empty string for invalid UTF-8 on UTF-8 sites. Since WP 6.9, wp_check_invalid_utf8( $text, true ) replaces invalid byte sequences with U+FFFD instead of silently removing them.

Use it when you are already in a WordPress escaping/sanitizing path that expects this helper. For new explicit validation logic, prefer wp_is_valid_utf8() and wp_scrub_utf8() because the intent is clearer.

Noncharacters

Unicode noncharacters can be valid UTF-8 while still being inappropriate for interchange formats. Use wp_has_noncharacters() when producing XML, strict external API payloads, or data that will be consumed outside WordPress.

if ( wp_has_noncharacters( $text ) ) {
    return new WP_Error(
        'myplugin_noncharacter_text',
        __( 'The text contains Unicode noncharacters that cannot be exported.', 'myplugin' )
    );
}

Critical rules

• Do not use `seems_utf8()` in new code. It is deprecated as of WP 6.9.
• Do not scrub identifiers. Reject invalid bytes for slugs, tokens, IDs, and security-sensitive values.
• Do not scrub too early. Replacement is lossy and may destroy useful import/debug information.
• Validate before serialization boundaries. XML, JSON, feed, sitemap, and external API payloads should not receive invalid bytes.
• Remember ASCII ambiguity. A string can be valid UTF-8 and still originate from a non-UTF-8 encoding if it contains only ASCII.

Common mistakes

// WRONG - deprecated and less explicit.
if ( ! seems_utf8( $value ) ) {
    $value = '';
}

// RIGHT - clear validation.
if ( ! wp_is_valid_utf8( $value ) ) {
    return new WP_Error( 'invalid_utf8', __( 'Invalid text encoding.', 'myplugin' ) );
}

// WRONG - silently changes an identifier.
$slug = sanitize_key( wp_scrub_utf8( $raw_slug ) );

// RIGHT - reject bad bytes before deriving identifiers.
if ( ! wp_is_valid_utf8( $raw_slug ) ) {
    return new WP_Error( 'invalid_slug_encoding', __( 'Invalid slug encoding.', 'myplugin' ) );
}
$slug = sanitize_key( $raw_slug );

Cross-references

• Run `wp-security-audit` when invalid text comes from uploads, REST, AJAX, or third-party APIs.
• Run `wp-rest-api` when text is accepted or returned through REST schemas.
• Run `wp-html-api` when text is being inserted into HTML fragments.

What this skill does NOT cover

• Full charset conversion from legacy encodings such as Windows-1250 or ISO-8859-2.
• Browser-side text handling.
• Translation/i18n placeholder correctness.

References

• WordPress 6.9 UTF-8 dev note: <https://make.wordpress.org/core/2025/11/18/modernizing-utf-8-support-in-wordpress-6-9/>
• UTF-8 helpers: wp-includes/utf8.php
• wp_check_invalid_utf8() / seems_utf8(): wp-includes/formatting.php