There are many reasons for using an API documentation. The easiest beeing able to see where you've missed a part of your phpdoc. There are also many solutions for documentation generation. I use ApiGen. They seem to be the most modern one. Here is how to use it:

Install apigen globally with

$ composer global require apigen/apigen 

No you can use it with:

$ ~/.composer/vendor/bin/apigen 

To be able to use it without the path, just put ~/.composer/vendor/bin into your PATH variable. Then you're able to run it with:

$ apigen 

Now create the file apigen.neon with the following content in your project root:

title: My API documentation
source:
    - src
destination: documentation
todo: true

Run apigen in your project root folder and it will create the documenation into your project root /documentation folder. You can find all available options here.