Description:
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There already exists __destruct(), but if you need to clean up mappings that use 
an object's identifier to cache results etc, there isn't a smooth way of doing 
this.

It would be nice to tie a callback to an object's lifespan.

The catch would of course be that you can't reference the object in the callback 
because of circular references, or maybe you can, I'm not that intimate with 
PHP's garbage collection.

Test script:
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$x = (object)[];
register_destructor($x,function($x){
    // extra cleanup operations with $x,
    // or instead an spl_object_hash() could be precalculated and closed over
    // if using $x would cause circular death
});