Create custom Action and Filter hooks for your plugins in WordPress

custom-wp-hooks.php

<?php
/**
 * Learn how to create custom Action and Filter hooks.
 *
 * @link  https://www.lynda.com/PHP-tutorials/Creating-custom-hooks/508212/547129-4.html
 */

/**
 * 1. Custom FILTER Hook ------------------------------
 */
function list_all_items() {
  
  $items = array( 
      'item 1',
      'item 2',
      'item 3',
  );
  
  $list = '<ul>';
  
  // If we have a custom filter hook, then call it here
  if ( has_filter( 'add_more_items' ) ) {
    $items = apply_filters( 'add_more_items', $items );
  }
  
  foreach ( $items as $item ) {
    $list .= '<li>' . $item . '</li>';
  }
  
  $list .= '</ul>';
  
  return $list;
  
}

// Custom Filter hook
add_filter( 'add_more_items', 'specify_filter_action' );
function specify_filter_action( $items ) {
  $items[] = 'item 4';
  return $items;
}

/**
 * 2. Custom ACTION Hook ------------------------------
 */

// 1. Define the action hook
function my_custom_action() {
  do_action( 'my_custom_action', 1 );
}

// 2. Attach a function to the hook
add_action( 'my_custom_action', 'specify_action' );
function specify_action() {
  echo 'Do something here.';
}

// 3. Call the action hook
my_custom_action();