Question

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What are PHP nested functions for?

PHP allows nested function declarations like so:

function foo() {
    function bar() {
        return "bar";
    }
    return "foo";
}

But what are the use cases for such a syntax? The inner bar is put to
global namespace so so it’s not garbage collected, is usable from outside
the outer function and provides a lot of confusion and possible bugs. For
example calling the foo twice results in an error, unless you wrap the
declaration inside a if(!is_callable('bar')).

Using nested declarations to create conditional declarations is not good
either, because of the confusion it creates. “Why is it complaining that
there is no such function, when it works perfectly fine in there?!?!”.

Answer 1

Well, conditional function declaration is probably the only legitimate use case, and there only to provide fallbacks for backwards compatibility:

function makeSureAllDependenciesExist() {
    if (!function_exists('someFunctionThatOnlyExistsInNewerPhpVersions')) {
        function someFunctionThatOnlyExistsInNewerPhpVersions() {
            // fallback implementation here
        }
    }
}

Other than that, there’s indeed little reason to use nested function declarations.