> react doing this very thing and calling it JSX

In PHP, “HTML" is string concatenation; in React, “HTML" is an HTML-flavoured bit of syntax sugar over HTML-generating functions (and you are still welcome to call the functions yourself, if you want your HTML-generation code to avoid looking like HTML). Aside from looking kind-of similar if you squint, I’d say the differences and pros and cons are fairly significant (For example the IMO worst thing about the PHP approach, the ease of shooting yourself in the foot with XSS holes, is a non-issue in JSX).