Description:
------------
Every time you create a new DOMDocument, even if you unset it directly afterwards, a small amount of memory is leaked.

Reproduce code:
---------------
<?php
while(true){
 $doc = new DOMDocument();
 unset($doc);
}
?>


Expected result:
----------------
Check the memory usage after a few seconds and notice it will go up dramatically. 

Actual result:
--------------
When checking how much memory is used by this process, it will go up with time, even though no variables are being left unset. This means that every time a new DOMDocument is constructed, it allocates more memory than it's destructor is handling. This can be very bad when creating lots of DOMDocuments within a single process.