Hi Derick,

the DateTimeImmutable class should have another static method:

create(DateTimeInterface $dt) {
  return $dt instanceof DateTimeImmutable
    ? $dt
    : new DateTimeImmutable($dt)
}

such a method is important in a constructor:

class MyClass {

  private $immutableDateTime;

  public function __construct(DateTimeInterface $dt) {
    $this->immutableDateTime = DateTimeImmutable::create($dt);
  }
}

You don't want somebody from outside to modify the internal state of MyClass.

I miss a direct way to cast from one to the other.

Too bad you are using such an example to justify DateTimeImmutable. The formatNextMondayFromNow() should clone the date:

function formatNextMondayFromNow( DateTime $dt )
{
    $dt = clone $dt;
    return $dt->modify( 'next monday' )->format( 'Y-m-d' );
}

Or better, be broken into two parts:

$now->next('monday')->format();

Where next() would return a new instance representing the next something, here "monday".

To me, with this example, it looks like DateTimeImmutable is a solution to bad usages.

I created an extension to DateTime and all the yesterday, monday, utc, local, … magic properties return new instances. Only modify() actually modifies the instance. I'm quite happy with it.

https://github.com/ICanBoogie/DateTime

@Thomas, @Per: I have added to PHP-5.6 a new method DateTimeImmutable::createFromMutable() that allows you to instantiate a non-mutable DateTimeImmutable variant based off a mutable DateTime object.

DateTimeImmutable::createFromMutable() is a good addition but conversely being able to do the opposite would have been great too DateTime::createFromImmutable()

This way switching from one class to the other would be less painfull.

Is it possible to make DateTime immutable in PHP 7 to avoid this madness?

@Daniel: Possible, but that is going to break quite a lot of code, and it's going to be difficult to spot/test. So that's not happening.

While we're at it. It would of course be nice to get rid of a mutable DateTime in PHP7 but I'm afraid that this would be too much of a break and Derick already commented on how important he considers backward compatibility.

But please add a method DateTimeImmutable::from(DateTimeInterface $dt).

This method is needed all the time when a Constructor accepts a DateTimeInterface. In this case one can either take a given DateTimeImmutable as is of create a new DateTimeImmutable from a given DateTime.

@Thomas: That method does exist:

• http://php.net/manual/en/datetimeimmutable.createfrommutable.php

And PHP 7 will have the createFromMutable method on DateTime too.