WordPress functions.php Template with 15 Essential Custom Functions (source:http://digwp.com/2010/03/wordpress-functions-php-template-custom-functions/)

functions.php

<?php // custom functions.php template @ digwp.com

// add feed links to header
if (function_exists('automatic_feed_links')) {
	automatic_feed_links();
} else {
	return;
}


// smart jquery inclusion
if (!is_admin()) {
	wp_deregister_script('jquery');
	wp_register_script('jquery', ("http://ajax.googleapis.com/ajax/libs/jquery/1/jquery.min.js"), false, '1.3.2');
	wp_enqueue_script('jquery');
}


// enable threaded comments
function enable_threaded_comments(){
	if (!is_admin()) {
		if (is_singular() AND comments_open() AND (get_option('thread_comments') == 1))
			wp_enqueue_script('comment-reply');
		}
}
add_action('get_header', 'enable_threaded_comments');


// remove junk from head
remove_action('wp_head', 'rsd_link');
remove_action('wp_head', 'wp_generator');
remove_action('wp_head', 'feed_links', 2);
remove_action('wp_head', 'index_rel_link');
remove_action('wp_head', 'wlwmanifest_link');
remove_action('wp_head', 'feed_links_extra', 3);
remove_action('wp_head', 'start_post_rel_link', 10, 0);
remove_action('wp_head', 'parent_post_rel_link', 10, 0);
remove_action('wp_head', 'adjacent_posts_rel_link', 10, 0);


// add google analytics to footer
function add_google_analytics() {
	echo '<script src="http://www.google-analytics.com/ga.js" type="text/javascript"></script>';
	echo '<script type="text/javascript">';
	echo 'var pageTracker = _gat._getTracker("UA-XXXXX-X");';
	echo 'pageTracker._trackPageview();';
	echo '</script>';
}
add_action('wp_footer', 'add_google_analytics');


// custom excerpt length
function custom_excerpt_length($length) {
	return 20;
}
add_filter('excerpt_length', 'custom_excerpt_length');


// custom excerpt ellipses for 2.9+
function custom_excerpt_more($more) {
	return '...';
}
add_filter('excerpt_more', 'custom_excerpt_more');

/* custom excerpt ellipses for 2.8-
function custom_excerpt_more($excerpt) {
	return str_replace('[...]', '...', $excerpt);
}
add_filter('wp_trim_excerpt', 'custom_excerpt_more'); 
*/


// no more jumping for read more link
function no_more_jumping($post) {
	return '<a href="'.get_permalink($post->ID).'" class="read-more">'.'Continue Reading'.'</a>';
}
add_filter('excerpt_more', 'no_more_jumping');


// add a favicon to your 
function blog_favicon() {
	echo '<link rel="Shortcut Icon" type="image/x-icon" href="'.get_bloginfo('wpurl').'/favicon.ico" />';
}
add_action('wp_head', 'blog_favicon');


// add a favicon for your admin
function admin_favicon() {
	echo '<link rel="Shortcut Icon" type="image/x-icon" href="'.get_bloginfo('stylesheet_directory').'/images/favicon.png" />';
}
add_action('admin_head', 'admin_favicon');


// custom admin login logo
function custom_login_logo() {
	echo '<style type="text/css">
	h1 a { background-image: url('.get_bloginfo('template_directory').'/images/custom-login-logo.png) !important; }
	</style>';
}
add_action('login_head', 'custom_login_logo');


// disable all widget areas
function disable_all_widgets($sidebars_widgets) {
	//if (is_home())
		$sidebars_widgets = array(false);
	return $sidebars_widgets;
}
add_filter('sidebars_widgets', 'disable_all_widgets');


// kill the admin nag
if (!current_user_can('edit_users')) {
	add_action('init', create_function('$a', "remove_action('init', 'wp_version_check');"), 2);
	add_filter('pre_option_update_core', create_function('$a', "return null;"));
}


// category id in body and post class
function category_id_class($classes) {
	global $post;
	foreach((get_the_category($post->ID)) as $category)
		$classes [] = 'cat-' . $category->cat_ID . '-id';
		return $classes;
}
add_filter('post_class', 'category_id_class');
add_filter('body_class', 'category_id_class');


// get the first category id
function get_first_category_ID() {
	$category = get_the_category();
	return $category[0]->cat_ID;
}

?>

functions_v2.php

<?php

// Change admin logo
function custom_admin_logo() { 
echo '<style type="text/css"> 
#header-logo { background-image: url('.get_bloginfo('template_directory').'/images/admin_logo.png) !important; } 
</style>'; 
} 
add_action('admin_head', 'custom_admin_logo'); 

// Change Footer Text in WP Admin 
function remove_footer_admin () { 
echo 'Siobhan is Awesome. Thank you <a href="http://wordpress.org">WordPress</a> for giving me this filter.'; 
} 
add_filter('admin_footer_text', 'remove_footer_admin'); 

// Dynamic Copyright Date in Footer
function comicpress_copyright() { 
 
global $wpdb; 
$copyright_dates = $wpdb->get_results("
SELECT
YEAR(min(post_date_gmt)) AS firstdate,
YEAR(max(post_date_gmt)) AS lastdate
FROM
$wpdb->posts
WHERE
post_status = 'publish' 
"); 
$output = ''; 
 
if($copyright_dates) { 
$copyright = "© " . $copyright_dates[0]->firstdate; 
 
if($copyright_dates[0]->firstdate != $copyright_dates[0]->lastdate) { 
$copyright .= '-' . $copyright_dates[0]->lastdate; 
} 
$output = $copyright;
} 
 
return $output;
} 

// Then insert this into your footer:
// <?php echo comicpress_copyright(); ?> 


// Add favicon
<span style="font-weight: normal;"> </span> 
// add a favicon to your
function blog_favicon() { 
echo '<link rel="Shortcut Icon" type="image/x-icon" href="'.get_bloginfo('wpurl').'/favicon.ico" />'; 
} 
add_action('wp_head', 'blog_favicon'); 

// Add support for backgrounds
add_custom_background(); 

// Edit the Help dropdown
//hook loading of new page and edit page screens
add_action('load-page-new.php','add_custom_help_page'); 
add_action('load-page.php','add_custom_help_page'); 
function add_custom_help_page() { 
//the contextual help filter
add_filter('contextual_help','custom_page_help'); 
} 
function custom_page_help($help) { 
//keep the existing help copy
echo $help; 
//add some new copy
echo "<h5>Custom Features</h5>"; 
echo "<p>Content placed above the more divider will appear in column 1. Content placed below the divider will appear in column 2.</p>"; 
} 

// Relative date
# For posts & pages #
<?php echo human_time_diff(get_the_time('U'), current_time('timestamp')) . ' ago'; 
# For comments #
<?php echo human_time_diff(get_comment_time('U'), current_time('timestamp')) . ' ago'; ?>

// Remove Title from WordPress Menu Linksfunction my_menu_notitle( $menu ){ 
 
return $menu = preg_replace('/ title="(.*?)"/', '', $menu ); 
} 
add_filter( 'wp_nav_menu', 'my_menu_notitle' ); 
add_filter( 'wp_page_menu', 'my_menu_notitle' ); 
add_filter( 'wp_list_categories', 'my_menu_notitle' ); 

// Breadcrumbs without a plugin 
function the_breadcrumb() { 
echo '<ul id="crumbs">'; 
 
if (!is_home()) { 
echo '<li><a href="'; 
echo get_option('home'); 
echo '">'; 
echo 'Home'; 
echo "</a></li>"; 
 
if (is_category() || is_single()) { 
echo '<li>'; 
the_category(' </li><li> '); 
 
if (is_single()) { 
echo "</li><li>"; 
the_title(); 
echo '</li>'; 
} 
} elseif (is_page()) { 
echo '<li>'; 
echo the_title(); 
echo '</li>'; 
} 
} 
 
elseif (is_tag()) {single_tag_title();} 
 
elseif (is_day()) {echo"<li>Archive for "; the_time('F jS, Y'); echo'</li>';} 
 
elseif (is_month()) {echo"<li>Archive for "; the_time('F, Y'); echo'</li>';} 
 
elseif (is_year()) {echo"<li>Archive for "; the_time('Y'); echo'</li>';} 
 
elseif (is_author()) {echo"<li>Author Archive"; echo'</li>';} 
 
elseif (isset($_GET['paged']) && !empty($_GET['paged'])) {echo "<li>Blog Archives"; echo'</li>';} 
 
elseif (is_search()) {echo"<li>Search Results"; echo'</li>';} 
echo '</ul>'; 
} 

// Insert into header.php
//<?php the_breadcrumb(); ?>


// Disable standard widgets
function unregister_default_wp_widgets() { 
unregister_widget('WP_Widget_Calendar'); 
unregister_widget('WP_Widget_Search'); 
unregister_widget('WP_Widget_Recent_Comments'); 
} 
add_action('widgets_init', 'unregister_default_wp_widgets', 1); 

// Google Analytics Without Editing Theme 
<?php 
 
add_action('wp_footer', 'ga'); 
function ga() { ?> 
// Paste your Google Analytics code here
<?php } ?> 

// Add Custom User Contact Info
/* BEGIN Custom User Contact Info */ 
function extra_contact_info($contactmethods) { 
unset($contactmethods['aim']); 
unset($contactmethods['yim']); 
unset($contactmethods['jabber']); 
$contactmethods['facebook'] = 'Facebook'; 
$contactmethods['twitter'] = 'Twitter'; 
$contactmethods['linkedin'] = 'LinkedIn'; 
 
return $contactmethods; 
} 
add_filter('user_contactmethods', 'extra_contact_info'); 
/* END Custom User Contact Info */ 
Then use this code wherever you want to display it:

 
<a href="<?php the_author_meta('facebook', $current_author->ID); ?>"></a> 



// Disable WordPress Search
function fb_filter_query( $query, $error = true ) { 
 
if ( is_search() ) { 
$query->is_search = false; 
$query->query_vars[s] = false; 
$query->query[s] = false; 
// to error
>if ( $error == true ) 
$query->is_404 = true; 
} 
} 
add_action( 'parse_query', 'fb_filter_query' ); 
add_filter( 'get_search_form', create_function( '$a', "return null;" ) ); 


// Add Custom Content Under Each Post 
function add_post_content($content) { 
 
if(!is_feed() && !is_home()) { 
$content .= '<p>This article is copyright © '.date('Y').' '.bloginfo('name').'</p>'; 
} 
 
return $content;
} 
add_filter('the_content', 'add_post_content'); 

// Display Latest Posts 
<?php query_posts('showposts=5'); ?> 
<ul> 
<?php while (have_posts()) : the_post(); ?> 
<li><a href="<?php the_permalink() ?>"><?php the_title(); ?></a></li> 
<?php endwhile;?> 
</ul> 

?>