The first part is brilliant. The explanations are clear, the examples very good and the learning curve almost perfect. When getting into Monoids and Monads it becomes harder to follow, but still quite good.
The part about practical monads, especially Reader, Writer and Io monads, is disappointingly superficial, with barely any example and, as a result, very hard to understand.
I glanced over the Laravel/Symfony integration and completely ignored Wordpress/Drupal, and didn't go too deep into the testing part (it's mostly quite logical).
If it weren't for the monads chapters, it would have deserved 5 stars.
A very good introduction to functional programming in PHP and, in my case, to the world of functional programming. I will not say this book was an easy read, because functional programming itself is a complex topic, but I believe the author made a really good job at describing the principles about this paradigm and a way that it can be approached in the PHP language. A must-read for any programmer wanting to learn functional programming in PHP.