====== Интересная идея - как реализовать функцию кэширования ======
<code PHP>
class CacheTest {

    private $cache = [];

    public function set($key, $value) {
        $this->cache[$key] = $value;
    }

    public function getOrNull($key) {
        return $this->cache[$key] ?? null;
    }

    public function getOrCall($key, $callback) {
        if (isset($this->cache[$key])) {
            return $this->cache[$key];
        } else {
            return $this->cache[$key] = $callback();
        }
    }

}

$cache = new CacheTest();
[$str1, $str2] = ["alive", "dead"];
$value1 = $cache->getOrCall('mordor', function() use ($str1) {
    return "Mordor is $str1!";
});

$value2 = $cache->getOrCall('mordor', function() use ($str2) {
    return "Mordor is $str2!";
});


header('Content-Type: text/plain; charset=UTF-8');
print_r([$value1, $value2]);
</code>