如何进行PHP后端功能开发中的代码重构？

function calculateArea($radius) {
    $pi = 3.14;
    $area = $pi * pow($radius, 2);
    return $area;
}

$area1 = calculateArea(5);
$area2 = calculateArea(7);

function calculateArea($radius) {
    $pi = 3.14;
    $area = $pi * pow($radius, 2);
    return $area;
}

$area1 = calculateArea(5);
$area2 = calculateArea(7);

class Database {
    private static $instance;

    private function __construct() {
        // 连接数据库
    }

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

$db1 = Database::getInstance();
$db2 = Database::getInstance();

class User {
    private $name;
    private $age;

    public function __construct($name, $age) {
        $this->name = $name;
        $this->age = $age;
    }

    public function getName() {
        return $this->name;
    }

    public function getAge() {
        return $this->age;
    }

    public function isAdult() {
        if ($this->age >= 18) {
            return true;
        } else {
            return false;
        }
    }
}

class User {
    // 属性和构造函数省略

    public function isAdult() {
        return $this->age >= 18;
    }
}