Some useful WP tricks

functions.php

<?php

/** Hide admin bar */
add_filter('show_admin_bar', '__return_false');

/** Custom favicon for debug mode (for easier difference) */
function debug_favicon(){
    if (WP_DEBUG) {
        echo '<link rel="icon" type="image/png" href="/favicon-production.png">';
    }
}
add_action('admin_head', 'debug_favicon');
add_action('wp_head', 'debug_favicon');

/** Turn on thumbnails */
add_theme_support('post-thumbnails');

/** Turn on menu */
function register_my_menu() {
    register_nav_menus(array(
        'menu' => 'Menu'
    ));
}
add_action('init', 'register_my_menu');

/** Empty search string fix */
function my_request_filter($query_vars) {
    if(isset($_GET['s']) && empty($_GET['s'])) {
        $query_vars['s'] = " ";
    }
    return $query_vars;
}
add_filter('request', 'my_request_filter');

/** Set first and last <li> classes in menus */
function add_first_and_last($items) {
    $parents = array();
    foreach ($items as $key => $item) {
        if (! array_key_exists($item->menu_item_parent, $parents)) {
            $item->classes []= 'first-menu-item';
        } else {
            array_pop($items[$parents[$item->menu_item_parent]]->classes);
        }
        $parents[$item->menu_item_parent] = $key;
        $item->classes []= 'last-menu-item';
    }
    return $items;
}
add_filter('wp_nav_menu_objects', 'add_first_and_last');

/** Add "attachment" class to images in WP editor */
function add_class_attachment_link($html){
    $html = str_replace('<a ', '<a class="attachment" ', $html);
    return $html;
}
add_filter ('media_send_to_editor', 'add_class_attachment_link');

/** Remove default size&cols from CF7 forms */
function remove_default_size($output) {
    $output = preg_replace('/size=\"40\"/', '', $output);
    $output = preg_replace('/cols=\"40\"/', '', $output);
    return $output;
}
add_filter('wpcf7_form_elements', 'remove_default_size');

/** Remove spaces between <li> in menus */
function remove_spaces($output) {
    $output = preg_replace('#<\/li>\s<li#iu', '</li><li', $output);
    return $output;
}
add_filter('wp_nav_menu', 'remove_spaces');

/** Gets post cat slug and looks for single-[cat slug].php and applies it */
function single_template_filter ($template) {
    foreach(get_the_category() as $cat) {
        if (file_exists(TEMPLATEPATH . "/single-{$cat->slug}.php")) {
            return TEMPLATEPATH . "/single-{$cat->slug}.php";
        }
    }
    return $template;
}
add_filter('single_template', 'single_template_filter');

function pre ($array) {
    echo '<pre>';
    print_r($array);
    echo '</pre>';
}

/** Smart excerpt for content */
mb_internal_encoding('UTF-8');
function smart_excerpt ($content, $symbols = 200) {
    if (mb_strlen($content) < $symbols) {
        return $content;
    }
    $content = mb_substr($content, 0, $symbols);
    $content = mb_substr($content, 0, mb_strrpos($content, ' '));
    $content .= '...';
    return $content;
}

/** Add alts to images depends on the title */
function add_image_alts($content) {
    // Remove alt that already exists
    $content = preg_replace('#<img(.*?)(alt=\"(.*?)\")(.*?)>#iu', "<img$1$4>", $content);
    // Add new alt depends on the title
    $content = preg_replace('#<img(.*?)>#iu', "<img$1 alt=\"{$GLOBALS['post']->post_title}\">", $content);
    return $content;
}
add_filter('the_content', 'add_image_alts');
add_filter('post_thumbnail_html', 'add_image_alts');

/** Remove width & height images attributes */
function remove_images_width_height($content) {
    $content = preg_replace('/<img(.*?)(width=\"\d+\")(.*?)>/iu', "<img$1 $3>", $content);
    $content = preg_replace('/<img(.*?)(height=\"\d+\")(.*?)>/iu', "<img$1 $3>", $content);
    return $content;
}
add_filter('the_content', 'remove_images_width_height');
add_filter('post_thumbnail_html', 'remove_images_width_height');

/** Breadcrumbs */
function breadcrumbs() {

    $text['home']     = 'Главная'; // текст ссылки "Главная"
    $text['category'] = '%s'; // текст для страницы рубрики
    $text['search']   = 'Результаты поиска по запросу "%s"'; // текст для страницы с результатами поиска
    $text['tag']      = 'Записи с тегом "%s"'; // текст для страницы тега
    $text['author']   = 'Статьи автора %s'; // текст для страницы автора
    $text['404']      = 'Ошибка 404'; // текст для страницы 404

    $show_current   = 1; // 1 - показывать название текущей статьи/страницы/рубрики, 0 - не показывать
    $show_on_home   = 0; // 1 - показывать "хлебные крошки" на главной странице, 0 - не показывать
    $show_home_link = 1; // 1 - показывать ссылку "Главная", 0 - не показывать
    $show_title     = 1; // 1 - показывать подсказку (title) для ссылок, 0 - не показывать
    $delimiter      = ' &rarr; '; // разделить между "крошками"
    $before         = '<span class="current">'; // тег перед текущей "крошкой"
    $after          = '</span>'; // тег после текущей "крошки"

    global $post;
    $home_link    = home_url('/');
    $link_before  = '';
    $link_after   = '';
    $link_attr    = '';
    $link         = $link_before . '<a' . $link_attr . ' href="%1$s">%2$s</a>' . $link_after;
    $parent_id    = $parent_id_2 = isset($post) ? $post->post_parent : 0;
    $frontpage_id = get_option('page_on_front');

    if (is_home() || is_front_page()) {

        if ($show_on_home == 1) echo '<span class="current">' . $text['home'] . '</span>';

    } else {

        if ($show_home_link == 1) {
            echo sprintf($link, $home_link, $text['home']);
            if ($frontpage_id == 0 || $parent_id != $frontpage_id) echo $delimiter;
        }

        if ( is_category() ) {
            $this_cat = get_category(get_query_var('cat'), false);
            if ($this_cat->parent != 0) {
                $cats = get_category_parents($this_cat->parent, TRUE, $delimiter);
                if ($show_current == 0) $cats = preg_replace("#^(.+)$delimiter$#", "$1", $cats);
                $cats = str_replace('<a', $link_before . '<a' . $link_attr, $cats);
                $cats = str_replace('</a>', '</a>' . $link_after, $cats);
                if ($show_title == 0) $cats = preg_replace('/ title="(.*?)"/', '', $cats);
                echo $cats;
            }
            if ($show_current == 1) echo $before . sprintf($text['category'], single_cat_title('', false)) . $after;

        } elseif ( is_search() ) {
            echo $before . sprintf($text['search'], get_search_query()) . $after;

        } elseif ( is_day() ) {
            echo sprintf($link, get_year_link(get_the_time('Y')), get_the_time('Y')) . $delimiter;
            echo sprintf($link, get_month_link(get_the_time('Y'),get_the_time('m')), get_the_time('F')) . $delimiter;
            echo $before . get_the_time('d') . $after;

        } elseif ( is_month() ) {
            echo sprintf($link, get_year_link(get_the_time('Y')), get_the_time('Y')) . $delimiter;
            echo $before . get_the_time('F') . $after;

        } elseif ( is_year() ) {
            echo $before . get_the_time('Y') . $after;

        } elseif ( is_single() && !is_attachment() ) {
            if ( get_post_type() != 'post' ) {
                $post_type = get_post_type_object(get_post_type());
                $slug = $post_type->rewrite;
                printf($link, $home_link . '/' . $slug['slug'] . '/', $post_type->labels->singular_name);
                if ($show_current == 1) echo $delimiter . $before . get_the_title() . $after;
            } else {
                $cat = get_the_category(); $cat = $cat[0];
                if ($cat->cat_ID != 1) { //Не показывать категорию "Без рубрики"
                    $cats = get_category_parents($cat, TRUE, $delimiter);
                    if ($show_current == 0) $cats = preg_replace("#^(.+)$delimiter$#", "$1", $cats);
                    $cats = str_replace('<a', $link_before . '<a' . $link_attr, $cats);
                    $cats = str_replace('</a>', '</a>' . $link_after, $cats);
                    if ($show_title == 0) $cats = preg_replace('/ title="(.*?)"/', '', $cats);
                    echo $cats;
                }
                if ($show_current == 1) echo $before . get_the_title() . $after;
            }

        } elseif ( !is_single() && !is_page() && get_post_type() != 'post' && !is_404() ) {
            $post_type = get_post_type_object(get_post_type());
            echo $before . $post_type->labels->singular_name . $after;

        } elseif ( is_attachment() ) {
            $parent = get_post($parent_id);
            $cat = get_the_category($parent->ID); $cat = $cat[0];
            $cats = get_category_parents($cat, TRUE, $delimiter);
            $cats = str_replace('<a', $link_before . '<a' . $link_attr, $cats);
            $cats = str_replace('</a>', '</a>' . $link_after, $cats);
            if ($show_title == 0) $cats = preg_replace('/ title="(.*?)"/', '', $cats);
            echo $cats;
            printf($link, get_permalink($parent), $parent->post_title);
            if ($show_current == 1) echo $delimiter . $before . get_the_title() . $after;

        } elseif ( is_page() && !$parent_id ) {
            if ($show_current == 1) echo $before . get_the_title() . $after;

        } elseif ( is_page() && $parent_id ) {
            if ($parent_id != $frontpage_id) {
                $breadcrumbs = array();
                while ($parent_id) {
                    $page = get_page($parent_id);
                    if ($parent_id != $frontpage_id) {
                        $breadcrumbs[] = sprintf($link, get_permalink($page->ID), get_the_title($page->ID));
                    }
                    $parent_id = $page->post_parent;
                }
                $breadcrumbs = array_reverse($breadcrumbs);
                for ($i = 0; $i < count($breadcrumbs); $i++) {
                    echo $breadcrumbs[$i];
                    if ($i != count($breadcrumbs)-1) echo $delimiter;
                }
            }
            if ($show_current == 1) {
                if ($show_home_link == 1 || ($parent_id_2 != 0 && $parent_id_2 != $frontpage_id)) echo $delimiter;
                echo $before . get_the_title() . $after;
            }

        } elseif ( is_tag() ) {
            echo $before . sprintf($text['tag'], single_tag_title('', false)) . $after;

        } elseif ( is_author() ) {
            global $author;
            $userdata = get_userdata($author);
            echo $before . sprintf($text['author'], $userdata->display_name) . $after;

        } elseif ( is_404() ) {
            echo $before . $text['404'] . $after;
        }

        if ( get_query_var('paged') ) {
            if ( is_category() || is_day() || is_month() || is_year() || is_search() || is_tag() || is_author() ) echo ' (';
            echo __('Page') . ' ' . get_query_var('paged');
            if ( is_category() || is_day() || is_month() || is_year() || is_search() || is_tag() || is_author() ) echo ')';
        }

    }
}

Is it typo in the name of func degub_favicon()?

Yeah, it is. Fixed