php手动释放内存

<?php
$s = str_repeat('1',1024);
$b = &$s;

$m=memory_get_usage(); //获取当前占用内存
echo $m."\n";

//unset($s);
//$s = null;

$mm = memory_get_usage(); //unset()后再查看当前占用内存
echo $mm . "\n";

echo $m -$mm ;
echo "\n";
?>