哪位能给下面这段php代码写个详细分析，最好具体到每一行。

Question

<?php
final class Flash {

const FLASHES_KEY = '_flashes';

private static $flashes = null;

private function __construct() {
}

public static function hasFlashes() {
    self::initFlashes();
    return count(self::$flashes) > 0;
}

public static function addFlash($message) {
    if (!strlen(trim($message))) {
        throw new Exception('Cannot insert empty flash message.');
    }
    self::initFlashes();
    self::$flashes[] = $message;
}

public static function getFlashes() {
    self::initFlashes();
    $copy = self::$flashes;
    self::$flashes = array();
    return $copy;
}

private static function initFlashes() {
    if (self::$flashes !== null) {
        return;
    }
    if (!array_key_exists(self::FLASHES_KEY, $_SESSION)) {
        $_SESSION[self::FLASHES_KEY] = array();
    }
    self::$flashes = &$_SESSION[self::FLASHES_KEY];
}

}

?>

Answer 1

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