If you are building any kind of API then you now, or at least you should now, that limiting the number of requests is one of the most important parts of your system. You don't want to allow anyone to hammer your endpoints without any kind of limitations. The more popular your app is becoming, the more you have to care about protecting your API endpoints.
Most PHP frameworks today have a concept of the middleware. The middleware is a layer that stands between the user request and your app, and this is the perfect place to put the request limiting mechanism. Before any request reaches your app, it has to pass through middleware. To count the number of incoming request per user you must have some sort of storage to persist data, and this storage has to be fast, in order to avoid slowing down your app.
NoSQL databases are growing in popularity every day, and because their key/value nature they are the perfect storage for request limiting layer. Couchbase is one of the most popular NoSQL databases today, and if you are creating an app that uses this database, then using it also to store request data is a perfect solution. My recommendation is to create a separate bucket for storing temporary data, in order to avoid mixing it with your business data.
I created a simple middleware that counts the number of incoming requests from IP addresses per unit of time. An IP address is saved as a key, and the number of requests from that address is an integer value. When you combine this, with a Couchbase document that has an expiration date (so-called time to live - TTL), you get a simple but powerful and fast rate limiter. This package was originally written for integration with the Slim framework, but you could use it with your custom project or with other frameworks. To install the package, use Composer:
composer require gnikolovski/cb-rate-limiter