PHP 函数如何扩展应用范围？

function compare($a, $b) {

  return $a - $b;

}

function sort_array($array, $compare_function) {

  usort($array, $compare_function);

}

$array = [1, 4, 2, 3];

sort_array($array, 'compare');

function getAreaCalculator($radius) {

  return function() use ($radius) {

    return pi() * $radius ** 2;

  };

}

$calculateArea = getAreaCalculator(5);

$area = $calculateArea();

$nextFibonacci = function($n) {

  return $n <= 1 ? $n : $nextFibonacci($n - 1) + $nextFibonacci($n - 2);

};