PHP memory leak in unserialize() test script

<?php

function generateRandomString($length = 10) {
    $characters = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
    $randomString = '';
    for ($i = 0; $i < $length; $i++) { $randomString .= $characters[rand(0, strlen($characters) - 1)]; }
    return $randomString;
}

class obj {
    public $items = array();
    public $data;
}

function subobj($obj, $depth=5) {
        for($i = 0; $i < 5; $i++) {
                $obj->items[$i] = new obj;
                $obj->items[$i]->data = generateRandomString(64);
        if($depth>1) subobj($obj->items[$i], $depth-1);
        }
}

print "Populating object...";
$obj = new obj;
for($i = 0; $i < 10; $i++) {
    subobj($obj);
}
print "\nSerializing...";

$data = serialize($obj);

print "\nLooping...\n";
$iterations = 20000;
while($iterations--) {
    $newstr = unserialize($data);
    if($iterations % 1000 == 0)
        print "$iterations: " . sprintf('%0.2f', memory_get_usage()/1024/1024) . "Mb\n";
}