I've been hearing that "PHP does not scale" arguement for many, many, years, in many, many, variations.
When you really have to scale, infrastructure, resources, and networks will rapidly overtake any concerns around your choice of language. You can develop crappy architectures in any language, and the inverse is true.
Not my experience. At scale, once you go down the 'cloud native' (kubernetes) path, the infrastructure part becomes relatively straight-forward, but PHP's weaknesses become very apparent.
Doesn't work very well with message queues. Sure, pushing messages is not a problem, but consuming? Something has to do it? But it won't be written in PHP. It doesn't work very well with SQL databases, and with that I mean not the querying etc, but connection pooling, so the database server doesn't suddenly gets flooded by thousands of connections because a service is autoscaled due to heavier load (and sadly, that's a real-world example that brought down a database cluster). Tracing is doable, but not ideal, metrics are hacky due to the need of weird extensions to support shared memory, or rely on an external database such as Redis, which kinda defeats the point of metrics being lightweight. And tuning the PHP runtime almost rivals the JRE...
When you really have to scale, infrastructure, resources, and networks will rapidly overtake any concerns around your choice of language. You can develop crappy architectures in any language, and the inverse is true.