...Yes? For one thing, PHP code runs
inside the process of rendering HTML, but JS SSR runs under the process of a React JS app, running in Node. That alone is an absolutely massive difference. Not just on a technical level; React JSX also doesn't have nearly as much hazards surrounding HTML sanitization, for example, because JSX is a DSL that protects inputs with proper sanitization by default, among other practical differences. Another difference is that SSR is more flexible than for runtime environment; it is very often used as a static compilation technique, where it is run as a compilation step for a static site, but either way, it is not married to any specific runtime model, whereas PHP pretty much is (for now.)
A closer analogue to what React does would be comparing XHP[1] to JSX, but even that misses the isomorphic aspect of a SPA with SSR, which is almost the entire value proposition, because it allows you to only need to write your component code once while reusing the same render cycle at browser runtime.
It should be pretty evident that PHP-style CGI scripting is not ideal given that even early on many PHP apps moved to using template systems like Smarty and never actually embedding HTML directly into PHP code.
[1]: https://github.com/phplang/xhp