Building a REST API in PHP doesn't require a heavy framework, but it does require discipline around consistency. Here are the practices that make the biggest difference.

Use proper HTTP status codes. Return 200 for success, 201 when something is created, 400 for bad input, 401/403 for auth failures, 404 when a resource doesn't exist, and 500 only for genuine server errors. Returning 200 with an error message buried in the JSON body forces every client to parse the body just to know if something worked.

Structure responses consistently. Pick a shape — for example { "success": true, "data": {...} } or { "success": false, "message": "..." } — and use it everywhere. A client integrating with your API should never have to guess the response shape based on which endpoint they called.

Version your API from day one. Even a simple /api/v1/ prefix saves you from breaking every existing client the first time you need to change a response format. Adding versioning after you have real consumers is far more painful than starting with it.

Validate input before touching the database. Check required fields, types, and formats before running any queries, and return a clear 400 response listing what's wrong. Vague failures like a generic 500 error make integrating with your API frustrating to debug from the outside.

Use PDO with prepared statements for every query. This matters even more for APIs, since they're often the most directly exposed part of your application to arbitrary input, including from clients you don't control.

Add rate limiting for public endpoints. Even a simple per-IP request counter stored in the database or a cache layer prevents one misbehaving client from degrading the service for everyone else.

Document as you build. A short README listing each endpoint, expected parameters, and a sample response saves enormous time later, both for you and for anyone else consuming the API. It doesn't need to be fancy — a markdown file is enough to start.