PHP - Why is DateTime object copied by reference in my code?

Question

Why in this code my DateTime object was copied by reference it seems?
Here's my code:

<?php

date_default_timezone_set('UTC');

$dt1 = new \DateTime('2015-03-15');
$dt2 = $dt1;
$dt2 = $dt2->modify('-1 year');

echo $dt1->format('c') . PHP_EOL;
echo $dt2->format('c');

?>

I was expecting:

2015-03-15T00:00:00+00:00
2014-03-15T00:00:00+00:00

But I got this:

2014-03-15T00:00:00+00:00
2014-03-15T00:00:00+00:00

If you use the class `DateTimeImmutable` it will work, it behaves exactly the same as `DateTime` class except it never modifies itself but returns a new object instead. — Daan, Dec 15 '14 at 15:22

score 8 · Answer 1 · edited May 23 '17 at 12:33

8

It's because of this line

$dt2 = $dt1;

Variables get copied, objects get referenced.

See this for an answer with examples - https://stackoverflow.com/a/6257203/1234502

You should be able to fix this with clone

edited May 23 '17 at 12:33

Community

1
1

answered Dec 15 '14 at 15:19

Pankucins

1,690
1
16
25

`Variables get copied, objects get referenced`, not really precise – mcont Dec 15 '14 at 15:21
Really? I thought that this is what happens in php. – Pankucins Dec 15 '14 at 15:24
4

Yes, but "variables" and "objects" aren't different things. You should say value/raw types and objects (as written in the answers you mentioned) – mcont Dec 15 '14 at 15:25
You are correct, read what I read once more and realized it isn't stating what I meant, at all. – Pankucins Dec 15 '14 at 15:29
4

Instead of just saying "not really precise" maybe try be more helpful to explain *why* off the bat? – LeonardChallis Jul 14 '16 at 09:44

score 0 · Answer 2 · answered Dec 15 '14 at 15:21

Consider the following text from PHP's Objects and references page:

As of PHP 5, an object variable doesn't contain the object itself as value anymore. It only contains an object identifier which allows object accessors to find the actual object.

Basically, your $dt2 = $dt1; is simply copying the object reference and not its contents; see the response by @lolka_bolka for the appropriate means by which to perform this task.

PHP - Why is DateTime object copied by reference in my code?

2 Answers2