The PHP manual states:

As of 8.0.0, the GD extension uses objects instead of resources, and objects cannot be explicitly closed.

This is useful, but also can be a little disingenuous – implying that image resources / objects can not be destroyed.

As mentioned on other questions there are some replacement methods that can be used for images. However, specifically looking at objects we can view this Q&A.

There are two useful methods to “clear” this memory footprint of the GD image object.

1.

Clear memory immediately. Use = null. Slight CPU load but suitable for loops dealing with significant numbers of images:

foreach($imageArray as $imageRow){
    $image = imagecreatefrompng($imageRow);

    // Do some stuff with $image

}
// Previously
// imagedestroy($image); 
// Now: 
$image = null;

2.

Clear memory efficiently. Use unset(). Suitable for effective cleaning large image resources in low number of images:

foreach($imageArray as $imageRow){
    $image = imagecreatefrompng($imageRow);

    // Do some stuff with $image

}
// Previously
// imagedestroy($image); 
// Now: 
unset($image);

Remember all image resources and objects are cleaned anyway automatically at the end of the script so if you’re dealing with non-massive images or very small numbers of images there’s little benefit to explicitly clearing the GD object before the natural end of the script.