Wordpress PHP function which adds CSS class to currently selected archive list item

theme_get_archives_link.php

// http://wordpress.org/support/topic/wp_get_archives-highlight-current-archive-please

/* Add a class to the archive link if it is the current one
   so we can, e.g. highlight it */

function theme_get_archives_link( $link_html ) {
    	
   global $wp;

   static $current_url;
		
    if( empty( $current_url ) ) {
        $current_url = add_query_arg( $_SERVER['QUERY_STRING'], '', home_url( $wp->request) );
    }

    if ( stristr( $link_html, $current_url )  ) {
        $link_html = preg_replace( '/(<[^\s>]+)/','\1 class="current-arch"',$link_html,1 );
    }
        
    return $link_html;
}

/* and of course you need to add the following hook */
add_filter('get_archives_link', 'theme_get_archives_link');

Thanks for the code, it helped me a lot.