Most PHP codebases simply don't have much use for IIFE's. A well-maintained project will be neatly organized into namespaces and classes, and undefined variable

Most PHP codebases simply don't have much use for IIFE's. A well-maintained project will be neatly organized into namespaces and classes, and undefined variables don't automatically become global. Meanwhile, PSR-1 says that a file that defines classes and/or functions should not have any side effect. So you don't have to worry about accidentally interacting with other scopes. Anonymous functions and classes are being used more and more, but PHP isn't fundamentally event-driven like JS so there's less need to hand them out like candy and nest them several levels deep. Most of the closures I do use on a daily basis are callback functions to things like array_filter() and preg_replace_callback(), not event handlers like every jQuery code ever written.