PHP
Data types
http://www.w3schools.com/php/php_arrays_multi.asp
WordPress
Conditional tags
https://codex.wordpress.org/Conditional_Tags
Functions
https://codex.wordpress.org/Template_Tags/get_posts
https://codex.wordpress.org/Function_Reference/query_posts
https://codex.wordpress.org/Function_Reference/set_transient
https://codex.wordpress.org/Function_Reference/get_transient
Classes
https://codex.wordpress.org/Class_Reference/WP_Query
API
https://codex.wordpress.org/Transients_API
Assignment
Create a plugin that adds a “You might also like…” block at the bottom of each post. Draw from other similar posts to create your suggestions. Bonus points for developing a weighted points sytem to determine the best matches.
Since these sorts of queries can be expensive operations, store the resulting recommendations in a transient for a reasonable amount of time.