Question 246426 in PHP Question posted in
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PHP date->modify and date_add. Why results of this two functions are the same?

I wrote this function

        $today=new DateTime(date('Y-m-d'));
        $tomorrow=new DateTime();
        $tomorrow=$today->modify('+1 day');
        
        $interval= new DateInterval('P7D');
        $enDt= date_add($today,$interval);
        var_dump($tomorrow->format('d-m-Y'), $enDt->format('d-m-Y'));

and I expected $tomorrow should be different from $enDt, because, in the first case, I increase $today of 1 day, and in the second case $enDt of 7 , but in real-world result is $enDT equals $tomorrow.

this is another version, with same result:

        $today=new DateTime(date('Y-m-d'));
        $tomorrow=new DateTime();
        $interval1= new DateInterval('P1D');
        $tomorrow=date_add($today,$interval1);

        $interval2= new DateInterval('P7D');
        $enDt= date_add($today,$interval2);
        var_dump($tomorrow,$enDt);
        var_dump($tomorrow->format('d-m-Y'), $enDt->format('d-m-Y'));
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2

Answers


  1. 0

    The DateTime class is mutable. It means that, when you call modify() or add() on it, the object is modified. So, in your example, $today is modified.

    You can use DateTimeImmutable instead of DateTime to work with immutable dates.

    $today = new DateTimeImmutable(date('Y-m-d'));
$tomorrow = $today->modify('+1 day');
        
$interval = new DateInterval('P7D');
$enDt = $today->add($interval);
var_dump($tomorrow->format('d-m-Y'), $enDt->format('d-m-Y'));

    Result:

    string(10) "17-09-2023"
string(10) "23-09-2023"
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  2. 0

    This is down to the way that DateTime works, as you effectively create 2 references to the same object ($today) you will end up changing the same value and end up with the same results.

    If you instead create a new date for the second date, you will have two separate objects and

    $today = new DateTime(date('Y-m-d'));
$tomorrow = $today->modify('+1 day');

$interval = new DateInterval('P7D');
$today2 = new DateTime();
$enDt = date_add($today2, $interval);
var_dump($tomorrow->format('d-m-Y'), $enDt->format('d-m-Y'));

    gives
    a.php:8:
    string(10) "17-09-2023"
    a.php:8:
    string(10) "23-09-2023"

    DateTimeImmutable is a good way of stopping this as well, BUT, date_add will not work with this (as of now).

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