We can use Swagger to generate API Documentation.
From Swagger.io "Swagger is a simple yet powerful representation of your RESTful API. With the largest ecosystem of API tooling on the planet, thousands of developers are supporting Swagger in almost every modern programming language and deployment environment. With a Swagger-enabled API, you get interactive documentation, client SDK generation and discoverability."
Demo : http://petstore.swagger.io/
PHP Library : https://github.com/zircote/swagger-php