PHP : exiftool using shell_exec -filemodifydate not applying (but is in bash)

Question

Using PHP to run exiftool on an image.

My code is :

// $newEpoch is defined by user input
$date = date('Y:m:d G:i:s', $newEpoch);
shell_exec("exiftool '-DateTimeOriginal={$date}' '-FileModifyDate={$date}' '{$file}'");

This sets the DateTimeOriginal tag (Date Taken in Windows Explorer) perfectly, but it doesn't apply the FileModifyDate.

If I run exiftool '-FileModifyDate=someDate' someFilePath" in a terminal, it works perfectly.

So why won't the PHP do it ?

I have no errors in the logs...

EDIT :

PHP is running in docker (container is php:apache8.2), host is a Raspberry Pi 4 running Debian.