[swsieber]

I'd take it one step further:

At Facebook scale, performance gains have huge returns (adding typing can improve pefromance), so they try to improve all parts of their infrastructure, not just PHP.

For example, Facebook's bolt for native code: https://code.fb.com/data-infrastructure/accelerate-large-sca...

PHP probably did have more low hanging fruit than say C, I'll give them that.