php中用unset销毁变量并释放内存

<?php
$s = str_repeat('1',255);  //产生由255个1组成的字符串
$m = memory_get_usage();  //获取当前占用内存
unset($s);
$mm = memory_get_usage();  //unset()后再查看当前占用内存
echo $m-$mm;
?>

<?php
$s = str_repeat('1',256);  //产生由256个1组成的字符串
$m = memory_get_usage();  //获取当前占用内存
unset($s);
$mm = memory_get_usage();  //unset()后再查看当前占用内存
echo $m-$mm;
?>

<?php
$s = str_repeat('1',256);    //这和第二个例子完全相同
$p = &$s;
$m = memory_get_usage();
unset($s);           //销毁$s
$mm = memory_get_usage();
echo $p . '<br />';
echo $m-$mm;
?>

<?php
$s = str_repeat('1', 256);   //这和第二个例子完全相同
$p = &$s;
$m = memory_get_usage();
$s = null;           //设置$s为null
$mm = memory_get_usage();
echo $p . '<br />';
echo $m-$mm;
?>

<?php
$s = str_repeat('1', 256);   //这和第二个例子完全相同
$p = &$s;
$m = memory_get_usage();
unset($s);           //销毁$s
unset($p);
$mm = memory_get_usage();
echo $p . '<br />';
echo $m-$mm;
?>

<?php
$arr = array("朝阳区","海淀区","西城区","东城区","丰台区");
unset($arr[3]);
echo "<pre>";
print_r($arr);
?>

Array
(
 [0] => 朝阳区
 [1] => 海淀区
 [2] => 西城区
 [4] => 丰台区
)