Your choice of backend and DB has nothing to do with the choice of your library or framework on the front-end.

I've always coded with what I know and it has worked for me - if you know PHP, go ahead and build your backend with PHP. If your data is relational and going to be suited for SQL, go for it by all means.

There are terminologies like MEAN and MERN going around, but I think that's just because someone who knew Mongo, Express, Angular and Node made it work together. Nothing's gonna stop you from using PHP, Laravel and MYSQL, so you can go ahead and call it a PLM stack.

Long story short, there's no single answer for the 'best' language on the backend to work with for react. 'Best' is not something that's tangible, everyone has their own definition. So, work with whatever language/framework you are comfortable with.

And, lastly, Welcome to Hashnode, Muhoho Jeremy. :D

As already pointed out by Siddarthan Sarumathi Pandian their is no best language, its all about which language you are comfortable with.

But, Using Express (Node) over other languages will give you advantage of processing the same code over server if you want to server render some of your functionality (For SEO, Fast first load).

So, if you are learning and no specific preference of any language go with Express (Node) for a React applications and you will realize certain benefits it has specially for JavaScript (Angular, React) based applications.

For DB, you can go as per your requirement. If the application has high volume of transactions and less reads ( Range of millions ) RDBMS databases will work fine. If you have high volume of data and reads (+ most data is of same format ) go with noSQL based DB's (MongoDB, CouchDB).