How deep is copy on write?

Hi folks. I have a question about the PHP runtime that I hope is appropriate for this list. (If not, please thwap me gently; I bruise easily.) I know PHP does copy-on-write. However, how "deeply" does it copy when dealing with nested arrays? This is probably easiest to explain with an example... $a['foo']['bar']['baz'] = 1; $a['foo']['bar']['bob'] = 1; $a['foo']['bar']['narf'] = 1; $a['foo']['poink']['narf'] = 1; function test($b) { // Assume each of the following lines in isolation... // Does this copy just the one variable baz, or the full array? $b['foo']['bar']['baz'] = 2; // Does this copy $b, or just $b['foo']['poink']? $b['foo']['poink']['stuff'] = 3; return $b; } // I know this is wasteful; I'm trying to figure out just how wasteful. $a = test($a); test() in this case should take $b by reference, but I'm trying to determine how much of a difference it is. (In practice my use case has a vastly larger array, so any inefficiencies are multiplied.) --Larry Garfield