Does it deterministically pass ALL tests in its own test suite yet? 100% of the time? This was the state just a few years ago, where a lot of tests were simply disabled because they were "flagging". <insert eyeroll emoji> (I'm not even going to touch the Boolean chart showing the different behaviors of single, double, and triple-equals on results. I know a lot of that has been fixed, at least by "convention".)
Nondeterminism is literally the worst trait of a computer language, because it leads to the worst kinds of bugs- the ones that are NOT a result of your lack of understanding of how the code is supposed to work at the PHP level. I've literally burned months away in my career in tracking down bugs like this (in Ruby and other languages), which ended up being framework or language bugs. There comes a point when you say "never again". Unfortunately, it comes too late in most people's careers to realize they've made a mistake, and switching to another language/community becomes too expensive/risky a proposition for them. (In my case I've been fortunate in that I was able to switch from .NET, to Ruby, to Elixir... each one of these was a difficult transition, both intellectually and socially (I really miss the Ruby community!), but in the end, completely justified in my case.)