PHP developers mainly create two types of applications: web apps with a database background, and APIs that provide your existing software and data to mobile apps and other developers. PHP code powers the back end of a web app, so a PHP developer will design and architect your code and the way it works to support the front end. APIs are often used when developers or apps need to query your internal data, and they allow you to provide this data to the outside world. For instance, your app might pull a list of tweets from Twitter’s API. A PHP developer will read the API documentation and implement it into your website. APIs are also used with some mobile apps to power the app’s data services.