php microtime_float 毫秒数获取

<?php

function microtime_float()

{

    list($usec, $sec) = explode(" ", microtime());

    return ((float)$usec + (float)$sec);

}

echo "<br />";

echo $time_start = microtime_float();

echo "<br />";

echo "time : ".microtime();

usleep(100);

echo "<br />";

echo $time_end = microtime_float();

echo "<br />";

echo "time : ".microtime();

$time = $time_end - $time_start;

echo "<br />";

echo "Did nothing in $time seconds\n";

结果如下：

1355455023.88
time : 0.87503500 1355455023
1355455023.88
time : 0.87547500 1355455023
Did nothing in 0.000448942184448 seconds

谁能帮我解答 下 最后 did nothing  in   0.000448942184448 seconds 是怎么回事？