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Date: Wed, 19 Jan 2011 00:45:31 -0600
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Message-ID: <201101190045.31781.larry@garfieldtech.com>
Subject: How deep is copy on write?
From: larry@garfieldtech.com (Larry Garfield)

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