PHP 8.6 passed its hard feature freeze on 22 September 2026, RC1 is due on 24 September, and general availability is scheduled for 19 November 2026. Its UPGRADING notes list these additions:
Partial function application: a new callable from a function with some arguments fixed
Default values on readonly properties
#[\Override] on class constants and enum cases, and __debugInfo() on enums
Direct mutation of objects held in constants, reference assignment into a WeakMap without the key existing first, and a new Time\Duration class
It also deprecates return inside a finally block, returning a value from __construct() or __destruct(), namespace as a class constant name, a function named readonly, and the third argument to define(). The code in this book uses none of them, so none of these deprecations will touch it when you move to 8.6. The book stays on 8.5 because it is the current stable series and in active support until 31 December 2027: moving to 8.6 is an option for 2027, not a requirement.