> For example, with the Qt library you can pass objects around by value, yet behind the scenes everything is reference counted with automatic copy-on-write. PH

> For example, with the Qt library you can pass objects around by value, yet behind the scenes everything is reference counted with automatic copy-on-write. PHP does exactly this, but for arrays and strings only (objects are implicitly passed by reference). So you can pass arrays by value with no performance penalty, as they are actually passed by reference. A COW mechanism ensures you end up with a local copy only if you write to an argument; such mechanism is disabled when passing arguments byref.