如何在PHP中通过类实现链式调用的详细步骤？

class Calculator {
    private $result = 0;

    public function add($number) {
        $this->result += $number;
        return $this; // 返回对象自身，实现链式调用
    }

    public function subtract($number) {
        $this->result -= $number;
        return $this;
    }

    public function multiply($number) {
        $this->result *= $number;
        return $this;
    }

    public function getResult() {
        return $this->result;
    }
}

// 使用链式调用
$calculator = new Calculator();
$result = $calculator->add(10)->subtract(5)->multiply(2)->getResult();
echo $result; // 输出 10

class DatabaseQuery {
    private $connection;
    private $query;
    private $error = null;

    public function __construct($connection) {
        $this->connection = $connection;
    }

    public function select($fields) {
        if ($this->error) return $this; // 如果之前有错误，直接返回

        $this->query = "SELECT " . implode(", ", $fields) . " FROM ";
        return $this;
    }

    public function from($table) {
        if ($this->error) return $this;

        $this->query .= $table;
        return $this;
    }

    public function where($condition) {
        if ($this->error) return $this;

        $this->query .= " WHERE " . $condition;
        return $this;
    }

    public function execute() {
        if ($this->error) return false;

        $result = mysqli_query($this->connection, $this->query);
        if (!$result) {
            $this->error = mysqli_error($this->connection);
            return false;
        }
        return $result;
    }

    public function getError() {
        return $this->error;
    }
}

// 使用示例
$db = mysqli_connect("localhost", "user", "password", "database");
$query = new DatabaseQuery($db);
$result = $query->select(['id', 'name'])->from('users')->where('status = 1')->execute();

if ($result) {
    // 处理结果
} else {
    echo "Error: " . $query->getError();
}