For instance, there are two websites - site1.com and site2.com. When a user visits site1.com, we need to find a way to remember this visit so that when the user goes to site2.com, we can accurately identify them as the same user. Basically, we need to leave some kind of identifier during the visit to site1.com (where could we put it?)

On the first domain, JavaScript can be used, but on the second site, the user's identification should happen at the PHP level immediately.

Without considering the IP address, it is initially unknown what the address of site2.com would be (when entering site1.com). Both sites can be hosted on the same hosting provider.

Is this feasible?

I personally don't endorse fingerprinting, so I'll present my own approach on how it can be done.

Here's an alternative solution: On site1.com, you can implement a JavaScript code (possibly using jQuery) that sends an XMLHttpRequest to site2.com. On site2.com, there should be a corresponding code to receive and process the user's request (preferably in JSON or a similar format). The key aspect is to ensure that the client-side request is not blocked and can synchronize between site1.com and site2.com.

As a result, site1.com will receive the user's request, and site2.com will also receive and store this request along with site1.com. Both sites can share a common database that is synchronized, and this database can be hosted on either site.

It's important to mention that synchronizing and sharing data between multiple websites requires careful consideration of security and privacy implications. Additionally, ensuring that the implementation adheres to legal regulations and best practices is crucial.

To achieve this, you can store the fingerprint data in the database of the first site. Then, on the second site, you can establish remote access to the database of the first site.

Creating a centralized database that can be accessed remotely from different sites can provide a way to maintain continuity and synchronize user information between multiple domains.

It is worth noting that ensuring the security and privacy of the stored data is of utmost importance. Implementing proper authentication measures and encryption protocols is crucial when dealing with sensitive user information across different sites.