The functions here are a compendium of snippets I've posted in the past. I will continue to update this file regularly.

/** ******** ******** ******** ******** ******** 
* TITLE: Google Analytics 4
* DESCRIPTION: Simplest way to add a Google Analytics Script without a Plug-in
*/
add_action('wp_head','my_analytics', 20);
function my_analytics() {
	?>          
<!-- Google tag (gtag.js) -->
<script async src="https://www.googletagmanager.com/gtag/js?id=G-xxxxxxxxxx"></script>
<script>
  window.dataLayer = window.dataLayer || [];
  function gtag(){dataLayer.push(arguments);}
  gtag('js', new Date());

  gtag('config', 'G-xxxxxxxxxx');
</script>

	<?php
}


/** ******** ******** ******** ******** ******** 
* Faster than @import
* 
* http://codex.wordpress.org/Customizing_the_Login_Form
* 
*/

function my_child_theme_scripts() {
    wp_enqueue_style( 'parent-theme-css', get_template_directory_uri() . '/style.css' );
}
add_action( 'wp_enqueue_scripts', 'my_child_theme_scripts' );



/** ******** ******** ******** ******** ******** 
* Customize Favicon
* 
* http://codex.wordpress.org/Customizing_the_Login_Form
* 
*/

// Custom Function to Include
function favicon_link() {
    echo '<link rel="shortcut icon" type="image/x-icon" href="' . get_stylesheet_directory() .'/favicons/favicon.ico" />' . "\n";
}
add_action( 'wp_head', 'favicon_link' );

/** ******** ******** ******** ******** ******** 
* Title: Remove Black Baar on mobile
*
* http://codex.wordpress.org/Roles_and_Capabilities#Subscriber
* http://docs.appthemes.com/tutorials/wordpress-check-user-role-function/
* 
* @param string $role Role name.
* @param int $user_id (Optional) The ID of a user. Defaults to the current user.
* @return bool
*/
if (!current_user_can('edit_posts') || wp_is_mobile() ) {
	show_admin_bar(false);
}

/** ******** ******** ******** ******** ******** 
* TITLE:       Change admin e-mail to new users
* DESCRIPTION: auto-detect the server so you only have to enter the front/from half of the email address, including the @ sign
* 
* http://snipplr.com/view/77687/wordpress-change-admin-email-to-new-users/
* http://miloguide.com/filter-hooks/wp_mail_from_name/
* http://premium.wpmudev.org/blog/wordpress-email-settings/
* 
*/
function admin_email_filter_wp_mail_from($email){
	/* start of code lifted from wordpress core, at 
	http://svn.automattic.com/wordpress/tags/3.4/wp-includes/pluggable.php */
	$sitename = strtolower( $_SERVER['SERVER_NAME'] );
	if ( substr( $sitename, 0, 4 ) == 'www.' ) {
		$sitename = substr( $sitename, 4 );
	}
	/* end of code lifted from wordpress core */
	$myfront = "noreply@";
	$myback = $sitename;
	$myfrom = $myfront . $myback;
	return $myfrom;
}
add_filter("wp_mail_from", "admin_email_filter_wp_mail_from");

/** ******** ******** ******** ******** ******** 
* TITLE:       Change admin name to new users
* DESCRIPTION: enter the full name you want displayed alongside the email address
* 
* http://miloguide.com/filter-hooks/wp_mail_from_name/
*/


function admin_name_filter_wp_mail_from_name($from_name){
	return "GuitarPick";
}
add_filter("wp_mail_from_name", "admin_name_filter_wp_mail_from_name");

/** ******** ******** ******** ******** ********
* TITLE: Beautify the Log-in Page
* DESCRIPTION: Make the Log-in Page PRetty
* 1: Change out Logo
* 2: Change out URL
* 3: Change out Title
* 
* http://codex.wordpress.org/Customizing_the_Login_Form
*/
function my_login_logo() { ?>
    <style type="text/css">
        body.login div#login h1 a {
            background-image: none;
        }
    </style>
<?php }
add_action( 'login_enqueue_scripts', 'my_login_logo' );

function my_login_logo_url() {
    return home_url();
}
add_filter( 'login_headerurl', 'my_login_logo_url' );

function my_login_logo_url_title() {
    return 'GuitarPick';
}
add_filter( 'login_headertitle', 'my_login_logo_url_title' );

function no_errors_please(){
  return 'Please Try Again';
}
add_filter( 'login_errors', 'no_errors_please' );

/** ******** ******** ******** ******** ******** 
* TITLE: Remove unnecessary meta-data from your WordPress site
* DESCRIPTION: Over the years, wordpress has placed a lot of junk in their system. 
*/
function remove_header_extra(){
	//Are you editing your WordPress blog using your browser? Then you are not using a blog client
	remove_action('wp_head', 'rsd_link');
	// Windows Live Writer is (it�s another blog editing client
	remove_action('wp_head', 'wlwmanifest_link');
	//This announces that you are running WordPress and what version you are using. 
	remove_action('wp_head', 'wp_generator');
	//URL shortening is sometimes useful, but this automatic ugly url in your header is useless.
	remove_action('wp_head', 'wp_shortlink_wp_head');
	// Display the links to the general feeds: Post and Comment Feed
	remove_action('wp_head', 'feed_links', 2);
	//// Display the links to the extra feeds such as category feeds
	remove_action('wp_head', 'feed_links_extra', 3);
	//Deprecated
	remove_action('wp_head', 'index_rel_link');
	// start link
	remove_action('wp_head', 'start_post_rel_link', 10, 0);
	// prev link
	remove_action('wp_head', 'parent_post_rel_link', 10, 0);
	// Display relational links for the posts adjacent to the current post.
	remove_action('wp_head', 'adjacent_posts_rel_link', 10, 0);
}
add_action('init', 'remove_header_extra');

/** ******** ******** ******** ******** ******** 
* Title: Wordpress Dashboard
* Description: Remove all those boxes on the Wordpress Dashboard
* 
* http://codex.wordpress.org/Function_Reference/remove_meta_box
*/
function remove_dashboard_meta() {
        remove_meta_box( 'dashboard_incoming_links', 'dashboard', 'normal' );
        remove_meta_box( 'dashboard_plugins', 'dashboard', 'normal' );
        remove_meta_box( 'dashboard_primary', 'dashboard', 'normal' );
        remove_meta_box( 'dashboard_secondary', 'dashboard', 'normal' );
        remove_meta_box( 'dashboard_incoming_links', 'dashboard', 'normal' );
        remove_meta_box( 'dashboard_quick_press', 'dashboard', 'side' );
        remove_meta_box( 'dashboard_recent_drafts', 'dashboard', 'side' );
        remove_meta_box( 'dashboard_recent_comments', 'dashboard', 'normal' );
        remove_meta_box( 'dashboard_right_now', 'dashboard', 'normal' );
        remove_meta_box( 'dashboard_activity', 'dashboard', 'normal' );
        remove_meta_box( 'dashboard_welcome_panel', 'dashboard', 'normal' );
}
add_action( 'admin_init', 'remove_dashboard_meta' );

/** ******** ******** ******** ******** ******** 
* Title: Limit The Excerpt's Word Count
*
* http://www.smashingmagazine.com/2011/12/07/10-tips-optimize-wordpress-theme/
* 
* 
*/
function ilc_excerpt_length( $length ){
  return 10;
}
add_filter('excerpt_length', 'ilc_excerpt_length');

/** ******** ******** ******** ******** ******** 
* Title: Remove Black Baar on mobile
*
* http://codex.wordpress.org/Roles_and_Capabilities#Subscriber
* http://docs.appthemes.com/tutorials/wordpress-check-user-role-function/
* 
* @param string $role Role name.
* @param int $user_id (Optional) The ID of a user. Defaults to the current user.
* @return bool
*/
if (!current_user_can('edit_posts') || wp_is_mobile() ) {
	show_admin_bar(false);
}

/** ******** ******** ******** ******** ******** 
* Title: Redirect Wordpress Feeds to Feedburner
*
* http://www.smashingmagazine.com/2011/12/07/10-tips-optimize-wordpress-theme/
*/
function feed_to_feedburner_redirect() {
  if ( is_feed() && !preg_match('/feedburner|feedvalidator/i', $_SERVER['HTTP_USER_AGENT'])){
    header('Location: http://feeds.feedburner.com/[THE_NAME_OF_YOUR_FEED]');
    header('HTTP/1.1 302 Temporary Redirect');
  }
}
//add_action('template_redirect', 'feed_to_feedburner_redirect');

/** ******** ******** ******** ******** ******** 
* Title: Remove fields from woocommerce checkout
* 
* [Override Billing](https://gist.github.com/mikejolley/1860056)
* [WooCommerce Plug-ins and hacks](http://wordpress.org/support/topic/woocommerce-remove-fields-on-edit-address)
* 
*/
 
// Our hooked in function - $fields is passed via the filter!
function custom_override_checkout_fields( $fields ) {
     unset($fields['order']['order_comments']);
     unset($fields['billing']['billing_phone']);
     unset($fields['billing']['billing_address_2']);
     unset($fields['billing']['billing_company']);
     unset($fields['shipping']['shipping_company']);
     unset($fields['shipping']['shipping_address_2']);

     return $fields;
}
add_filter( 'woocommerce_checkout_fields' , 'custom_override_checkout_fields' );

/** ******** ******** ******** ******** ******** 
* Title: Make Woocommerce Phone Field Not Required
* 
* [Override Billing](https://gist.github.com/mikejolley/1860056)
* [WooCommerce Plug-ins and hacks](http://wordpress.org/support/topic/woocommerce-remove-fields-on-edit-address)
* 
*/

function wc_npr_filter_phone( $address_fields ) {
  $address_fields['billing_phone']['required'] = false;
	return $address_fields;
}
add_filter( 'woocommerce_billing_fields', 'wc_npr_filter_phone', 10, 1 );

/** ******** ******** ******** ******** ******** 
* Title: Show Featured Images In Feed
* 
* http://www.smashingmagazine.com/2011/12/07/10-tips-optimize-wordpress-theme/
* 
* 
*/

function rss_post_thumbnail($content) {
  global $post;
  if( has_post_thumbnail($post->ID) )
    $content = '<p>' . get_the_post_thumbnail($post->ID, 'thumbnail') . '</p>' . $content;
  return $content;
}
add_filter('the_content_feed', 'rss_post_thumbnail');