What are PHP nested functions for?

If you are using PHP 5.3 you can get more JavaScript-like behaviour with an anonymous function:

<?php
function outer() {
    $inner=function() {
        echo "test\n";
    };

    $inner();
}

outer();
outer();

inner(); //PHP Fatal error:  Call to undefined function inner()
$inner(); //PHP Fatal error:  Function name must be a string
?>

Output:

test
test

There is none basically. I've always treated this as a side effect of the parser.

Eran Galperin is mistaken in thinking that these functions are somehow private. They are simply undeclared until outer() is run. They are also not privately scoped; they do pollute the global scope, albeit delayed. And as a callback, the outer callback could still only be called once. I still don't see how it's helpful to apply it on an array, which very likely calls the alias more than once.

The only 'real world' example I could dig up is this, which can only run once, and could be rewritten cleaner, IMO.

The only use I can think of, is for modules to call a [name]_include method, which sets several nested methods in the global space, combined with

if (!function_exists ('somefunc')) {
  function somefunc() { }
}

checks.

PHP's OOP would obviously be a better choice :)

[Rewritten according to the comment by @PierredeLESPINAY.]

It's not just a side-effect at all, but actually a very useful feature for dynamically modifying the logic of your program. It's from the procedural PHP days, but can come in handy with OO architectures too, if you want to provide alternative implementations for certain standalone functions in the most straightforward way possible. (While OO is the better choice most of the time, it's an option, not a mandate, and some simple tasks don't need the extra cruft.)

For example, if you dynamically/conditionally load plugins from your framework, and want to make the life of the plugin authors super easy, you can provide default implementations for some critical functions the plugin didn't override:

<?php // Some framework module

function provide_defaults()
{
    // Make sure a critical function exists:
    if (!function_exists("tedious_plugin_callback"))
    {
        function tedious_plugin_callback()
        {
        // Complex code no plugin author ever bothers to customize... ;)
        }
    }
}