Add GET params to links in HTML.

parametize_links.php

<?php

/**
 * Filters HTML adding URL parameters to links beginning with a base URL.
 *
 * The HTML source code is parsed with a regex replacement which updates the
 * "href" attribute of all links whose URL begins with the base URL, adding
 * the given data to the URL in the form of GET parameters.
 *
 * Glossary:
 *
 * parametize
 *     A fake term describing the action of adding parameters to something. 
 *
 * @param string $html     The HTML source to filter.
 * @param string $base_url The base URL to determine which links are filtered.
 * @param array  $data     An array mapping added parameter names to values.
 *
 * @return string
 */
function parametize_links($html, $base_url, $data)
{
    $url = preg_quote($base_url, '/');
    $re = '/
        (                           # Capture...
            <a[^>]*                 #   The a tag and all attributes until...
                href="' . $url . '  #   The href attribute with the base URL
                    [^"?]*          #   More URL until a question mark or quote
        )                           # End the capture leaving off...
            [?]?                    # The question mark maybe (but not a quote)
        /x';
    $sub = sprintf('\1?%s&', http_build_query($data));
    return preg_replace($re, $sub, $html);
}

?>