Chad Chad - 4 years ago 263
PHP Question

Deep copy of PHP array of references

So $array is an array of which all elements are references.

I want to append this array to another array called $results (in a loop), but since they are references, PHP copies the references and $results is full of identical elements.

So far, the best working solution is:

$results[] = unserialize(serialize($array));

which I fear to be incredibly inefficient. Is there a better way to do this?

Answer Source

You can use the fact that functions dereferences results when returning, for exemple here $array_by_myclone will still have a reference to $original ($array_by_myclone[0][0] == 'foo') while $array_by_assignment will have a cloned value ($array_by_assignment[0][0] == 'bar')

$original = 'foo';
$array_of_reference = array(&$original);

function myclone($value)
  return $value;

$array_by_myclone = array();
$array_by_myclone[] = array_map('myclone', $array_of_reference);

$array_by_assignment = array();
$array_by_assignment[] = $array_of_reference;

$original = 'bar';

var_dump($array_by_myclone[0][0]); // foo, values were cloned                                                                                                                                   
var_dump($array_by_assignment[0][0]); // bar, still a reference                     

EDIT: I wanted to check if the comment saying unserialize(serialize()) was faster was right so I did the test using php 5.5, and it turns out this is wrong: using the serialization method is slower even with a small dataset, and the more data you have the slower it gets.

lepidosteus@server:~$ php -v
PHP 5.5.1-1~dotdeb.1 (cli) (built: Aug  3 2013 22:19:30) 
Copyright (c) 1997-2013 The PHP Group
Zend Engine v2.5.0, Copyright (c) 1998-2013 Zend Technologies
    with Zend OPcache v7.0.2-dev, Copyright (c) 1999-2013, by Zend Technologies
lepidosteus@server:~$ php reference.php 1
myclone:   0.000010 seconds
serialize: 0.000012 seconds
lepidosteus@server:~$ php reference.php 1000000
myclone:   0.398540 seconds
serialize: 0.706631 seconds

Code used:

$iterations = 1000000;
if (isset($argv[1]) && is_numeric($argv[1])) {
  $iterations = max(1, (int)$argv[1]);

$items = array();
for ($i = 0; $i < $iterations; $i++) {
  $items[] = 'item number '.$i;

$array_of_refs = array();
foreach ($items as $k => $v) {
  $array_of_refs[] = &$items[$k];

function myclone($value)
  return $value;

$start = microtime(true);

$copy = array_map('myclone', $array_of_refs);

$time = microtime(true) - $start;

printf("%-10s %2.6f seconds\n", 'myclone:', $time);

$start = microtime(true);

$copy = unserialize(serialize($array_of_refs));

$time = microtime(true) - $start;

printf("%-10s %2.6f seconds\n", 'serialize:', $time);
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