DateTime Unexpected Mutation

Why This Happens

PHP's DateTime class is mutable, meaning methods like modify(), add(), and sub() change the original object. When you pass a DateTime to a function that modifies it, the original variable is also changed. This is a frequent source of bugs. DateTimeImmutable methods return new instances instead.

The Problem

$start = new DateTime('2024-01-01');
$end = $start->modify('+30 days'); // $start is also modified!
echo $start->format('Y-m-d'); // 2024-01-31, not 2024-01-01

The Fix

$start = new DateTimeImmutable('2024-01-01');
$end = $start->modify('+30 days'); // Returns new instance
echo $start->format('Y-m-d'); // 2024-01-01 (unchanged)
echo $end->format('Y-m-d');   // 2024-01-31

Step-by-Step Fix

1. 1

   Replace DateTime with DateTimeImmutable

   Change new DateTime() to new DateTimeImmutable() throughout your codebase to prevent accidental mutations.

2. 2

   Update type hints

   Change type hints from DateTime to DateTimeImmutable, or use DateTimeInterface to accept both.

3. 3

   Assign modify results

   When using DateTimeImmutable, always assign the result of modify(), add(), and sub() to a variable since they return new instances.