Challenge 1 — Solution

Task: Implement a Singleton class called Logger with a private array
property for storing log lines, a static getInstance() method, and a
public method log(string $message) that appends to the array.
Demonstrate that calling Logger::getInstance() from two different
points in the script still shares the same log entries.

<?php
    class Logger {
        private static ?Logger $instance = null;
        private array $lines = [];

        private function __construct() {}

        public static function getInstance(): Logger {
            if (self::$instance === null) {
                self::$instance = new self();
            }
            return self::$instance;
        }

        public function log(string $message) {
            $this->lines[] = $message;
        }

        public function getLines(): array {
            return $this->lines;
        }
    }

    // Point 1 in the script:
    Logger::getInstance()->log("Application started");

    // Point 2 in the script — a completely separate call:
    Logger::getInstance()->log("User logged in");

    print_r(Logger::getInstance()->getLines());
?>


Output:
Array
(
    [0] => Application started
    [1] => User logged in
)

Notes:
- Both "points" in the script call Logger::getInstance() independently,
  yet both log entries end up in the same array - proving both calls
  returned the exact same shared object, not two separate Logger
  instances.
- The constructor is private, exactly matching the chapter's Config
  example, so "new Logger()" is impossible from outside the class -
  getInstance() is the only way to obtain (or create, the first time)
  the one shared instance.
- getLines() is a small addition beyond the chapter's own Config
  example, added here purely so the accumulated log entries could be
  demonstrated and verified.