php依赖注入三种方式

class Database {
    private $connection;

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

    public function query($sql) {
        return $this->connection->query($sql);
    }
}

$db = new Database(new PDO('mysql:host=localhost;dbname=test', 'user', 'password'));

class Logger {
    public function log($message, $handler) {
        $handler->write($message);
    }
}

class FileHandler {
    public function write($message) {
        file_put_contents('log.txt', $message . PHP_EOL, FILE_APPEND);
    }
}

$logger = new Logger();
$handler = new FileHandler();
$logger->log('Hello, World!', $handler);

class EmailSender {
    public $transport;

    public function send($message) {
        $this->transport->send($message);
    }
}

class SmtpTransport {
    public function send($message) {
        // SMTP发送逻辑
    }
}

$sender = new EmailSender();
$sender->transport = new SmtpTransport();
$sender->send('Hello, World!');