Why ask it if you know it's stupid? No, PHP+Varnish can not always scale. You're forgeting the most important important factor in this equation, the database.

Why ask it if you know it's stupid? No, PHP+Varnish can not always scale. You're forgeting the most important important factor in this equation, the database. Varnish is simply for caching and you won't find any large site that doesn't have it. PHP is a scripting language and the runtime performance differences to other common web application languages (i.e. Python, Java) is negligible since you're using caching anyway. Here's what you should worry about if you want scaling: Your database, your data structures and your code quality.