MadCowWeb

//ADD BANNER TO CART PAGE
add_action('woocommerce_before_cart', 'madcow_above_cart_and_checkout_message');
add_action('woocommerce_after_order_notes', 'madcow_above_cart_and_checkout_message');
function madcow_above_cart_and_checkout_message() {
    echo 'your-content-here';
}

MadCowWeb

// CHECK FOR SPECIFIC PRODUCT CATEGORY IN CART
function madcow_custom_category_is_in_the_cart($categories) {
    // Products currently in the cart
    $cart_ids = array();
    // Categories currently in the cart
    $cart_categories = array();
    // Find each product in the cart and add it to the $cart_ids array
    foreach (WC()->cart->get_cart() as $cart_item_key => $values) {
        $cart_product = $values['data'];
        $cart_ids[]   = $cart_product->id;

MadCowWeb

// CHECK FOR SPECIFIC PRODUCT ID IN CART
function madcow_custom_product_id_is_in_the_cart($ids) {
    // Products currently in the cart
    $cart_ids = array();
    // Find each product in the cart and add it to the $cart_ids array
    foreach (WC()->cart->get_cart() as $cart_item_key => $values) {
        $cart_product = $values['data'];
        $cart_ids[]   = $cart_product->id;
    }
    // If one of the special products are in the cart, return true.

MadCowWeb

//ADD NEATO TEXT CUSTOM TAB
add_filter('woocommerce_product_tabs', 'madcow_neato_text_tab');
function madcow_neato_text_tab($tabs) {
    $tabs['neato_text_tab'] = array(
        'title'    => __('Our Neato Text', 'woocommerce'),
        'priority' => 5,
        'callback' => 'madcow_neato_text_tab_content',
    );
    return $tabs;
}

MadCowWeb

// move category above product title
remove_action('woocommerce_single_product_summary', 'woocommerce_template_single_meta', 40);
add_action('woocommerce_single_product_summary', 'woocommerce_template_single_meta', 4);

MadCowWeb

add_action('init', 'madcowweb_shortcodes');
function madcowweb_shortcodes() {
    add_shortcode('show-cows', 'show_cows');
}

//SIMPLE TEXT
function show_cows() {
    $html = '<h1>I love cows</h1>';
    return $html;
}