Surprise, no API here!

Write your own code or add any script, library or plugin you want.

Recommendations:

Medoo - The Lightest PHP database framework.
Put medoo.php into app folder and use it!

Bootstrap - The most popular front-end framework for developing responsive, mobile first projects on the web.
Put Bootstrat stuff into css and js folders and use it! Remember to update view files for Bootstrap layout.

Foundation - The most advanced responsive front-end framework in the world.
Put Foundation stuff into css and js folders and use it! Remember to update view files for Foundation layout.

Unheap - Best place to find cool jQuery plugins.
Put plugins into js folder and use them!

PHP on Github - find other useful PHP stuff.
Put PHP stuff into app folder and use them!