Getting Started with WordPress Plugin Development: The Ultimate Guide (2015 Jan): https://premium.wpmudev.org/blog/wordpress-plugin-development-guide/

How To Create A WordPress Plugin (2014 Jul): https://www.elegantthemes.com/blog/tips-tricks/how-to-create-a-wordpress-plugin

Plugin API/Action Reference/the post: https://codex.wordpress.org/Plugin_API/Action_Reference/the_post#Example

Actions Run During a Typical Request: https://codex.wordpress.org/Plugin_API/Action_Reference

One drawback of WordPress development is that WordPress was released before PHP 5, so much of the WordPress code is procedural and can be difficult to organize.

PHP WordPress Plugin Tutorial using Object Oriented Programming Part 1: Advanced Features – Pak PHP Micro Framework package blog: http://www.phpclasses.org/blog/package/9505/post/2-PHP-WordPress-Plugin-Tutorial-using-Object-Oriented-Programming-Part-1-Advanced-Features.html

Contents
Introduction
How to Translate your plugin
How to Create a Simple Settings Page for your Plugin
How to create a Dashboard Widget
How to Make AJAX Calls
How to Create Custom Post Types and Taxonomies
How to Unit Test Your Plugin by Extending the WordPress XML-RPC interface
How to include third party scripts
Conclusion

PHP WordPress Plugin Tutorial using Object Oriented Programming Part 1: Basic Example – Pak PHP Micro Framework package blog: http://www.phpclasses.org/blog/package/9505/post/1-PHP-WordPress-Plugin-Tutorial-using-Object-Oriented-Programming-Part-1-Basic-Example.html

WordPress and Laravel: https://laravel-news.com/2016/01/wordpress-and-laravel/

A popular way of setting up a site like this is to use WordPress as the admin and then build out the frontend in a framework such as Laravel.

WordPress Corcel (under DEVelopment)

This way you can use WordPress as back-end, to insert posts, custom types, etc, and you can use what you want in front-end, like Silex, Slim Framework, Laravel, Zend, or even pure PHP (why not?).

Using WordPress with Lumen: http://adampatterson.ca/blog/2015/06/using-wordpress-with-lumen/