Dmitry, is there any way to fix this..? (btw. unset() does NOT free all the memory..but that's, AFAIK, by design)

This is not really a bug, but unefficient usage of memory by zend_object_storage. It frees object buckets only on request shutdown.

The test case allocates:
16384 * sizeof(zend_object_store_bucket) = 393216 bytes

Note that the memory is freed inside php, just not yielded back to the operating system until execution ends.

In this example, the memory is claimed as the function executes, then re-used when the function is run again (instead of using up twice as much memory):
----------
<?php
function doNothing() {
	$res = array();
	for ( $i = 0; $i < 20000; $i++) {
		$res[$i] = new StdClass;
	}
}

// Unix only
function get_usage() {
	$my_pid = getmypid();
	$usage = `ps -eo%mem,rss,pid | grep $my_pid`;
	$kb = preg_split("/\s+/", $usage);
	return sprintf("%.3f",$kb[2] / 1024) . "MB";
}

echo "Before: ".get_usage()."\n";
doNothing();
echo "One Use: ".get_usage()."\n";
doNothing();
echo "Two Uses: ".get_usage()."\n";
?>

Output
------
Before: 5.555MB 
One Use: 10.164MB 
Two Uses: 10.293MB

Has this bug been fixed in PHP 5.*?

This is not a bug, and it's not fixed.

In PHP 7 the object store is just a ptr array, so the overhead here reduces to sizeof(void *) * max_number_of_objects. This should reduce the memory usage of the object store by a factor of 8 on both 32bit and 64bit.