I want to suggest something else entirely: Become accustomed to learning from non-book resources. First of all, the three things you have mentioned in your question that you want to learn more about differ substantially. Stream contexts are part of the core php API, and can be thought of as a "framework feature". Closures are a "language feature" (just another name for anonymous functions, really). Finally, process forking is more of an "OS feature", mostly available in *NIX flavor OS'es.
So what kind of book (with less than 1000s of pages) would include meaningful insights for these topics? I haven't read "Expert PHP and MySQL" but the name suggests that its focus is on something else.
So I want to suggest that you start reading the PHP language reference. It has all the details about all the topics there are. If some general computer science terms are unknown to you, you ask a question on SO or read up on it on wikipedia. If you find a topic in the reference that you don't understand or don't see the point of, you skip it, but make a mental note of the topic's existence so that you can return to it at a later time. The pro's of this approach are that you yourself can choose the focus of your study and do not have to rely on finding a book that covers your immediate interests. Furthermore, online documentation is usually more up-to-date than books, and online docs are also free. Granted, this approach takes a little getting used to and maybe also requires better analytical skills, but in the long run it is a much faster way to learn.
Afterall, in todays market, who has the time to wait for a book to arrive?