Description:
------------
Using a for loop to generate a range of dates and using the date modify to do the increment, the initial variable used for the start date is also being updated. I don't think this is the correct behaviour and certainly isn't consistent with a simple for loop.

Must admit I haven't tested in PHP 7.4.26 but have in 7.4.25

Tested using the 7.4.25 version of Xampp environment and also the 'try it' feature of the w3schools

Test script:
---------------
$start_date = new datetime('first Monday of January 2022');
$end_date = new datetime('last Monday of January 2022');

for ($date1 = $start_date; $date1 <= $end_date; $date1->modify('+1 week')) {
    echo $date1->format('Y-m-d').'<br>';
    }

echo 'Start date '. $start_date->format ('Y-m-d');

Gives
2022-01-03
2022-01-10
2022-01-17
2022-01-24
2022-01-31
Start date 2022-02-07

The original start date of 2022-01-03 has been updated to 2022-02-07 through the loop

Compare this to a simple for loop

$start = 5;

for ($x = $start; $x <= 10; $x++) {
  echo "The number is: $x <br>";
}
echo 'start '.$start;
?>  

Gives

The number is: 5
The number is: 6
The number is: 7
The number is: 8
The number is: 9
The number is: 10
start 5

This is as expected  

Expected result:
----------------
Should be -
For a for loop with date modify
Start 2022-01-03 (the original value)

Actual result:
--------------
Actual
For a for loop with date modify
Start date 2022-02-07 (the updated value at the end of the loop)