Cannot release variable’s memory with unset($var) or $var=null

I am trying to release the memory by useing unset() and $var = null;, but not obvious changes with just Only reduced a few KB

here is my code

var_dump(memory_get_usage()/1024/1024);   // output: 0.420059204102
$paginator = new Paginator( $mysql, $sql );

var_dump(memory_get_usage()/1024/1024);   // output: 0.420318603516

$ret = $paginator->getData('all', 1);
var_dump(memory_get_usage()/1024/1024);   // output: 80.0789031982

$form = $ret->items;
unset($paginator);
unset($ret);
// $paginator = null;
// $ret = null;
gc_collect_cycles();   // try to force free
var_dump(memory_get_usage()/1024/1024);   // output: 80.0779571533