How to use PHP Namespaces in WordPress Plugins | https://www.ibenic.com/php-namespaces-wordpress-plugins

api.php

<?php
namespace My_Plugin\API;

/**
 * API Functions
 */

if( ! defined( 'ABSPATH' ) ) {
    return;
}

/**
 * Get all users from an API
 * @return array
 */
function get_users() {
    $api_key = ''; //Should have a way to retrieve the api key.
    // API class is under namespace My_Plugin\API so we can call it directly.
    $api = new API();
    $users = $api->call( 'get_users_url', array( 'api_key' => $api_key ), 'get' );
    // Performs an API call.
    return $users;
}

class-api.php

<?php
namespace My_Plugin\API;

/**
 * API class
 */

if( ! defined( 'ABSPATH' ) ) {
    return;
}

class API {
    /**
     * Making a call
     * @param  string $url  
     * @param  array  $args    
     * @param  string $method  
     * @return WP_Error|WP_Http          
     */
    public function call( $url = '', $args = array(), $method = 'post' ) {
        if( 'post' === $method ) {
            return wp_remote_post( $url, $args );
        } else {
            return wp_remote_get( $url, $args );
        }
    }
}

db.php

<?php
namespace My_Plugin\DB;
/**
 * Database Functions
 */

if( ! defined( 'ABSPATH' ) ) {
    return;
}

/**
 * Get all users from a database
 * @return array 
 */
function get_users() {
    // Perform a database call to get users
    return array();
}

db2.php

<?php
namespace My_Plugin\DB;
use My_Plugin\API\API;

db3.php

<?php

/**
 * Showing how to call functions from others
 * @return API
 */
function populate_db_from_api() {
    $api = new API();
    return $api;
}

includes.php

<?php

// ...

class My_Plugin {

    /**
     * Loading all dependencies
     * @return void 
     */
    public function load() {
        include_once 'includes/api/class-api.php';
        include_once 'includes/db/functions.php';
        include_once 'includes/api/functions.php';
    }
}

loading.php

<?php

// ...

/**
 * Loading my plugin
 * @return void 
 */
function my_plugin_load() {
    $pg = new My_Plugin();
    $pg->load();
}

// We need to call the function with the namespace
add_action( 'plugins_loaded', 'My_Plugin\my_plugin_load' );

plugin.php

<?php
/**
 * Plugin Name: Namespaces in WordPress
 * Plugin Description: A showcase plugin so you can learn how to use namespaces in your projects.
 */

namespace My_Plugin;

if( ! defined( 'ABSPATH' ) ) {
    return;
}

class My_Plugin {
}

plugin2.php

<?php
// ...

class My_Plugin {
  // ...

  /**
   * Dummy method to check if we should use api or database call
   * @return bool 
   */
  public function use_api() {
      // check for api key or similar.
      return false;
  }

  /**
   * Get users
   * @return void 
   */
  public function get_users() {
      if( $this->use_api() ) {
          return API\get_users();
      } else {
          return DB\get_users();
      }
  }
}