Defines a clean interface for file storage operations (read, write, delete, list, etc.) with implementations for local filesystem and WebDAV. The Pro edition adds S3, SFTP, FTP, and cloud storage adapters. This interface is isolated and could be extracted into a standalone library for any PHP project needing pluggable storage backends.

Implements a granular per-folder permission system with inheritance, memoization, and atomic user grant updates. Supports buckets (owners, read, write, share, etc.) and subtree operations. The design is self-contained and demonstrates non-obvious skill in handling complex permission logic with file-based storage and concurrent safety.

Provides streaming file encryption and decryption using libsodium secretstream (XChaCha20-Poly1305). Handles large files efficiently with chunked processing and atomic writes. The class is isolated and could be reused in any PHP project requiring transparent encryption of file streams.

Implements a file-based rate limiter for TOTP attempts using flock for concurrency safety and HMAC-keyed identifiers to avoid leaking usernames/IPs. The pattern is reusable for any rate-limiting scenario where a database is not available, and demonstrates careful handling of atomic file operations.