You seriously aren't willing to admit that your question has zero relevance to someone working in PHP?
The entire point of coding in a language like PHP is that you don't think about the low level stuff. And even if you did take the time to truly understand the source of PHP and made some fabulous optimization, they could change implementation details in a future release and break what you did.
Bottom line is that your interview question has almost no correlation with whether someone would be a good PHP developer. You might as well ask what the capital of Macedonia is as that is just as good of a screen for potential applicants.
The entire point of coding in a language like PHP is that you don't think about the low level stuff. And even if you did take the time to truly understand the source of PHP and made some fabulous optimization, they could change implementation details in a future release and break what you did.
Bottom line is that your interview question has almost no correlation with whether someone would be a good PHP developer. You might as well ask what the capital of Macedonia is as that is just as good of a screen for potential applicants.