Copy-on-write \ Lazy Copy Mechanism

Hey, I started to learn for the Zend Certificate exam a few days ago and I got stuck with one unanswered-well question. This is the question: “Absent any actual need for choosing one method over the other, does passing arrays by value to a read-only function reduce performance compared to passing them by reference?’ The answers speaks about Lazy Copy Mechanism\Copy-on-Write. I tried to search at php.net and google.com about how this mechanism works, and unfortunately there were no results. I`m trying to understand what is the deference between passing array by value (when readonly) and passing array by reference, and why passing the array by value is faster? Thank you for your answer, Have a nice day, Shaked.