The following WordPress PHP code enforces lowercase URLs and trailing slashes. These have particular and distinctive SEO advantages, especially concerning how search engines crawl and index a webpage.

/**
 * Force redirection of uppercase URLs to lowercase and adding a slash at the end of the URL if it does not exist.
 *
 * @author Neat WP
 * @author_url https://neatwp.com
 */

add_action('template_redirect', 'force_lowercase_and_trailing_slash');
function force_lowercase_and_trailing_slash() {
    $request_uri = $_SERVER['REQUEST_URI'];
    $new_url = null;
    
    $parsed = parse_url($request_uri);
    $path = $parsed['path'] ?? '';
    $query = isset($parsed['query']) ? '?' . $parsed['query'] : '';

    if (preg_match('/[A-Z]/', $path)) {
        $path = strtolower($path);
        $new_url = $path . $query;
    }

    if ($path !== '' && !preg_match('/\.[a-z0-9]{2,6}\/?$/i', $path) && !preg_match('/\/$/', $path)) {
        $path_with_slash = $path . '/';
        $new_url = ($new_url === null) ? $path_with_slash . $query : str_replace($path, $path_with_slash, $new_url);
    }

    if ($new_url !== null && $new_url !== $request_uri) {
        wp_redirect($new_url, 301);
        exit;
    }
}

Below is a detailed explanation of the SEO purposes:

1. Preventing Duplicate Content

Problem: Search engines, with Google being the notable example, treat URLs of differing cases and/or with added trailing slashes as different pages (e.g., /Page, /page, /Page/, /page/). Hence, these become duplicate content in Google’s eyes, which disturbs a webpage’s ability to rank and confuses which is to be taken as the canonical one.

Solution: Keeping the code intact, all URL variations will be redirected to a single version: lowercase with a trailing slash (/page/). This 301 (permanent) redirect informs search engines about the preferred redirection and it consolidates link equity while avoiding penalties for duplicate content.

SEO-impact: Since the readers are being secured from duplicate URLs, focus on a single version of each page will then be converted into better horsepower and rankability for each page.

2. Crawl Budget Optimization

Problem: Search engines give local “crawl budgets” to each website, which entail the number of pages they crawl and index in a given period. If a site happens to have multiple URL variations for the same content (e.g., /About, /about, /About/), then search engine bots waste their crawl budget by indexing redundant pages and may very well miss the very content that needs to be indexed.

Solution: By redirecting all non-standard URLs to a single, consistent view of the URL (lowercase with trailing slash), the code reduces URLs that search engines need to crawl. In other words, rather than crawl /Page, /page, and /Page/, the bot only processes /page/.

SEO Impact: These aggressively conserve crawl budget, thereby enabling search engines to focus on crawling and indexing new or updated content, which is immensely important for large sites with thousands of pages and frequent content updates.

3. Improving Link Equity Consolidation

Problem: External and internal link variations across different URLs (e.g., /Contact vs. /contact/) are splitting link juice (the SEO value passed through those links) between multiple URLs, thus lessening the overall authority of the page.

Solution: Through 301 redirects, all links will be redirected to a single canonical URL (e.g. /contact/). For example, should an external site link to /Contact, the code will forward that link to /contact/, while giving the link’s SEO value to the canonical version.

SEO Impact: Consolidation of link equity establishes greater authority on the canonical URL and, thus, theoretically should improve its rankings on the search engine.

4. Enhancing Indexation Accuracy

Problem: Different URLs may cause search engines to index the less-preferred or wrong version of the page, misaligning with the site’s structured layout or user experience.

Solution: This code enforces standardized URL format (lowercase with trailing slash) for search engines to index the desirable version of every page. It also prevents search engines from indexing file URLs (such as /script.php) with unnecessary trailing slashes, which could give rise to errors or confusion.

SEO Impact: Proper indexing of the correct URL leads to higher visibility and display of the site as intended within search results, affirming the site as a trustworthy entity in the eyes of users who then go on to click on it.

5. Reducing Redirect Chains and Server Load

Problem: Various versions of a URL can be used to implement redirects (e.g., /PAGE → /Page → /page/), increased page load times, and server load, which both can eventually harm SEO.

Solution: The code performs redirects efficiently in one step by simultaneously checking for uppercase letters and missing trailing slashes, redirecting directly to the final URL (e.g., /PAGE → /page/).

SEO Impact: Faster redirects mean a better user experience and fewer wasted crawls, as search engines tend to favor sites that load fast with little-to-no redirect chains.

6. Supporting URL Structure Best Practices

Problem: Search engines and users often expect directory-like URLs to end with a trailing slash (e.g., /category/) and files to omit it (e.g., /image.jpg). Inconsistent URL structures always make the users and bots confused.

Solution: The code adds trailing slashes to non-file paths (excluding extensions such as .php, .jpg) while setting all other paths to lowercase, following SEO best practices for clean and predictable URLs.

SEO Impact: A consistent URL structure increases crawlability, user experience, search engine alignment, and the ranking.

7. Mitigating Case-Sensitivity Issues

Problem: A few web servers (e.g., Linux-based) operate in a case-sensitive manner, treating /Page and /page as two different URLs. If not handled well, this can lead to 404 errors and/or duplicate content issues.

Solution: The URL is forced lowercase by the code, solving that case-sensitivity issue and making sure requests always land on a valid and consistent URL.

SEO Impact: Preventing 404 errors and ensuring all URLs resolve correctly improves site reliability in the eyes of search engines, supporting better indexing and user satisfaction.

This code is a strategic SEO tool ensuring URL constancy to avoid duplicate content issues and preserve crawl budget. The lowercase URL with a trailing slash policy is best practice in the eyes of search engines; thus, it allows the site to benefit from authority and speeds up crawling. This contributes to higher rankings and stronger online presence. Crawl budget savings are valid especially if there are many pages and content gets updated often, leaving more chances for search engines to be able to index the most important content.