Is it safe to reuse the same \DateTime in a loop?

Question

I need to format several thousand Y-m-d H:i:s dates from an API response (sample). I used Xdebug to profile these two approaches.

What I have

foreach ($dates as $date) {
    $dt = new \DateTimeImmutable($date);
    $value = $dt->format('r');
}

Calls                               Count   Total Call Cost
php::DateTimeImmutable->__construct 55800   183 ms  1,79%
php::DateTimeImmutable->format      55800   14 ms   0,14%

What I tried

$dt = new \DateTime;
foreach ($dates as $date) {
    $dt->modify($date);
    $value = $dt->format('r');
}

Calls                       Count   Total Call Cost
php::DateTime->modify       55800   130 ms  1,10%
php::DateTime->format       55800   17 ms   0,14%
php::DateTime->__construct  1       0 ms    0,00%

The latter approach looks slightly more efficient. Is it safe to rely on just modifying the datetime instead of completely redeclaring it?

Does a difference of 53ms over 55800 items really warrant this…? — deceze, Oct 06 '20 at 07:46
IMHO, unless you have a strong reason to squeeze milliseconds you should stick to the code that's easier to understand. Your algorithm is not about altering a given date with an array of dates, yet that's what the `->modify()` version suggest at first sight. — Álvaro González, Oct 07 '20 at 15:20

score 0 · Answer 1 · answered Oct 06 '20 at 07:50

We're talking micro-optimizations, your example with DateTimeImmutable inside the loop is slower because you create a new DateTimeImmutable object each time. It's faster when you create the object outside of the loop obviously. and yes modifying it on each time is safe!

Is it safe to reuse the same \DateTime in a loop?

What I have

What I tried

1 Answers1