Use a small xml string and start this code
and look at your memory server, I've 150Mb
before the end of the loops :-(
Of course, I've the same result with a few loops
an big xml data.

for($j=1;$j<=20;$j++){
	for($i=1;$i<=1000;$i++){
		xmldoc($xml);
		echo "<br>loop $j:$i\n";
	}
	sleep(2);
}

Is it a memory leak?
Or a feature to add like xmldocfree()?