Why not simply using hiphop-php ? This is what empowers facebook and seem pretty neat https://github.com/facebook/hiphop-php
Try using HipHop before using the word "simple." Take it from me, as someone who's been playing with it inside of the last 72 hours, it's not an instant plug-in option, and it's absolutely not simple. You're compiling PHP scripts to C++, and if you use any non-standard modules, you're going to be in for some fun.
This option is simple: it's a plug-in module, just like APC, and it requires very little setup or configuration.
If you're going from using nothing to using Zend Optimizer+, you should be blown away by the difference it makes.
Honestly, I think that's a bit of a "how long is a piece of string" question. First step is to do zero scaling. As users come on board and you start hitting performance issues, work out where your problems lie - the database or filesystem rather than your code - and optimize for that.
Let's not forget the classic one either: your server might just be underpowered or lacking in RAM.
Once you get into significant numbers of users or usage, it'll quickly become apparent where the bottlenecks lie, and you should optimize as appropriate.
So going back to your question: when PHP becomes the bottleneck.
function f ($a, $k)
var_dump($hit);
In addition switching to HipHop is far more involved than optimizer modules like this. In my experience (back in the dreary days when I did PHP stuff) installing APC took just one apt-get and a restart of the web server for instant gains.
Adding the opcode cache to core will help get more eyes on the code. This should have happened sooner.
hphp was a program built from hiphop that compiled php to c++. This is being completely ripped out of hiphop.
hhvm is also built from the hiphop source. It is a JIT for php, and currently the main focus of the hiphop project.
The rest is still of course true, you cant just switch to hiphop it supports an incomplete set of php which is enough for facebook, but not the average library that you want to import via composer(also doesnt support composer, due to no namespaces yet).