A PHP REST API typically routes incoming requests by URL and HTTP method to controller logic, reads JSON from the request body via file_get_contents('php://input'), and responds with header('Content-Type: application/json') plus json_encode(). Proper use of HTTP status codes (200, 201, 400, 404, 422, 500) communicates outcomes to clients.
Frameworks like Laravel and Symfony handle routing, validation, and serialization out of the box, but understanding the raw mechanics — reading the request stream, setting headers before any output, and structuring consistent JSON error responses — is essential for debugging and for building lightweight APIs without a full framework.