PHP 8 Syntax speed difference?

Hey all. I discovered a weird performance thing about php 8.1.'s ?-> syntax.

I am serializing hundreds of objects into json-api format using Fractal. There are several layers down of relationships that it is serializing as well, and each object can have upwards of 20 fields.

I was trying to discover why serializing was slow and i discovered the following:

This:

            'created_at' => $item->created_at?->toIso8601String(),
            'updated_at' => $item->updated_at?->toIso8601String(),

was faster than this:

            'created_at' => empty($item->created_at) ? null : $item->created_at->toIso8601String(),
            'updated_at' => empty($item->created_at) ? null : $item->updated_at->toIso8601String(),

I tested and re-tested this a bunch of times; Fractal's serialization was about 300ms faster each time with the new syntax.

That seemed incredibly bizarre to me, so I ran a test in Tinkerwell:

$times = 10_000_000;

$startTime = microtime(true);
for ($i = 0; $i < $times; $i++){
  $carbon = new Carbon\Carbon();
  $save = empty($carbon) ? null : $carbon->toIso8601String();
}
$endTime = microtime(true);
echo (float)$endTime - (float)$startTime;

echo PHP_EOL;

$startTime = microtime(true);
for ($i = 0; $i < $times; $i++){
  $carbon = new Carbon\Carbon();
  $save = $carbon?->toIso8601String();
}
$endTime = microtime(true);
echo (float)$endTime - (float)$startTime;

And the ?-> syntax turned out to be slower by a few seconds!

Can anyone explain what's going on here? Why would using ?-> in fractal be faster but not in raw PHP?