Slim — Lightweight PHP Micro Framework for REST APIs

Introduction

Slim is a PHP micro framework designed for building APIs and small-to-medium web applications. It implements PSR-7 HTTP messages and PSR-15 middleware, giving developers a thin but standards-compliant layer between an HTTP request and response without the overhead of a full-stack framework.

What Slim Does

• Routes HTTP requests to callable handlers using a fast, tree-based router
• Implements PSR-7 request/response interfaces for interoperable HTTP handling
• Supports a middleware pipeline for cross-cutting concerns like auth and CORS
• Provides dependency injection via any PSR-11 compatible container
• Handles error rendering with customizable error handlers and formatters

Architecture Overview

Slim's core is a dispatcher that wraps a PSR-7 request through a middleware stack, matches the URI against registered routes, invokes the matched callable, and returns the PSR-7 response. The framework has no ORM, templating engine, or built-in auth — those are added via Composer packages, keeping the core under 70 KB.

Self-Hosting & Configuration

• Requires PHP 8.1+ and Composer
• Install with composer require slim/slim plus a PSR-7 implementation
• Entry point is typically public/index.php behind Apache or Nginx
• Configure via PHP arrays or a PSR-11 container; no YAML or INI files needed
• Deploy to any PHP-capable host, Docker, or serverless (AWS Lambda via Bref)

Key Features

• PSR-7 and PSR-15 compliance ensures interoperability with the PHP ecosystem
• Minimal footprint keeps startup time and memory usage low
• Flexible middleware architecture for layering functionality
• Route groups with shared middleware for clean API versioning
• Built-in support for content negotiation and response formatting

Comparison with Similar Tools

• Laravel — full-stack with ORM, queues, and auth; Slim is minimal and bring-your-own
• Symfony — enterprise component library; Slim is a single micro framework
• Lumen — Laravel-flavored micro framework; Slim is framework-agnostic and PSR-first
• CodeIgniter — lightweight but opinionated MVC; Slim has no view layer
• Mezzio (Laminas) — PSR-15 micro framework; Slim offers simpler API with less configuration

FAQ

Q: Is Slim suitable for large applications? A: Slim can power large apps when combined with a container, ORM, and templating engine, but a full-stack framework may be more productive at scale.

Q: How do I add templating? A: Install a renderer like slim/twig-view or slim/php-view and register it in the container.

Q: Does Slim support WebSockets? A: Not natively. Slim handles HTTP request-response cycles; use Ratchet or Swoole for WebSockets.

Q: What changed in Slim 4? A: Slim 4 decoupled the PSR-7 implementation, adopted PSR-15 middleware, and requires an explicit PSR-11 container.

Sources

• https://github.com/slimphp/Slim
• https://www.slimframework.com/docs/v4/