DateTime示例

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

• This will output:

    2021-05-15
    2021-05-15
  

• What happened here is that the modify returned the same instance of the DateTime object. The variable $tomorrow doesn't contain a different object, it contains a reference to the original one.

DateTimeImmutable示例

  $date = new DateTimeImmutable();
  $tomorrow = $date->modify('+1 day');
  echo $date->format('Y-m-d');
  echo $tomorrow->format('Y-m-d');	

• This will output:

    2021-05-14
    2021-05-15
  

• Because in DateTimeImmutable, the modification methods don't return the same instance, they give you a fresh one (also immutable).