PHP 函数的函数指针用于解决哪些具体问题？

<?php
// 回调函数
function greet($name) {
  echo "Hello, $name!\n";
}

// 调用回调函数
call_user_func('greet', 'John'); // 输出：Hello, John!
?>

<?php
class MyClass {
  // 添加事件监听器
  public function addEventListener($event, $listener) {
    $this->listeners[$event][] = $listener;
  }

  // 触发事件
  public function triggerEvent($event, $args) {
    foreach ($this->listeners[$event] ?? [] as $listener) {
      call_user_func($listener, $args);
    }
  }
}

$obj = new MyClass();
$obj->addEventListener('click', function() { echo 'Click event triggered!\n'; });
$obj->triggerEvent('click'); // 输出：Click event triggered!

<?php
// 比较函数
function compareAge($person1, $person2) {
  return $person1['age'] - $person2['age'];
}

// 对数组按年龄排序
$people = [
  ['name' => 'John', 'age' => 30],
  ['name' => 'Mary', 'age' => 25],
  ['name' => 'Bob', 'age' => 35],
];

usort($people, 'compareAge');

// 输出排序后的数组
foreach ($people as $person) {
  echo $person['name'] . ': ' . $person['age'] . "\n";
}
?>