DateTimeInterface::format() with Pure attribute

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[LolGleb]

Thank you for your PR! After reviewing it, I believe that adding the #[Pure] attribute to the DateTimeInterface::format() method is not appropriate. Here’s why:

This method depends not only on the input argument, but also on the internal state of the object.

For example, calling the method with the same $format value can yield different results depending on the instance’s internal state:

$date1 = new DateTime('2023-02-03');
$date2 = new DateTime('2024-04-05');
echo $date1->format('Y-m-d'); // "2023-02-03"
echo $date2->format('Y-m-d'); // "2024-04-05"

This violates one of the key conditions of purity: the output of the method must be consistent given the same input arguments.

If you think I am wrong, please point out where I am mistaken.

[LolGleb]

It turns out that for conditionally pure methods, where the method itself does not cause side effects, but its result may depend on the state of the object, you need to use the #[Pure(true)] attribute. This is exactly our case. Could you please make the changes in the PR?