PHP
Operators
https://secure.php.net/manual/en/language.operators.comparison.php
WordPress
Actions
https://codex.wordpress.org/Plugin_API#Actions
https://codex.wordpress.org/Function_Reference/add_action
https://codex.wordpress.org/Post_Status_Transitions
https://core.trac.wordpress.org/browser/tags/4.2.2/src/wp-includes/post.php#L3944
Classes
https://codex.wordpress.org/Class_Reference/WP_Post
https://codex.wordpress.org/Class_Reference/WP_User
Functions
https://codex.wordpress.org/Function_Reference/wp_get_current_user
https://codex.wordpress.org/Function_Reference/get_user_by
https://codex.wordpress.org/Function_Reference/wp_mail
Assignment
Create a plugin that emails the author of a published post when someone else trashes it. Bonus points for telling the author who did the dastardly deed.
This is meant to be a first pass at objects and classes, not in-depth exploration. You should be comfortable accessing properties of the WP_Post and WP_User objects.