原因：PHP底层是用快排实现的。无法保证元素的顺序。
所以要使用natsort（）；
搬运一个w3c的例子
$temp_files = array("temp15.txt","temp10.txt",
"temp1.txt","temp22.txt","temp2.txt");
sort($temp_files);
echo "标准排序：";
print_r($temp_files);
echo "<br>";
natsort($temp_files);
echo "自然排序：";
print_r($temp_files);结果:
标准排序：Array ( [0] => temp1.txt [1] => temp10.txt [2] => temp15.txt [3] => temp2.txt [4] => temp22.txt )
自然排序：Array ( [0] => temp1.txt [3] => temp2.txt [1] => temp10.txt [2] => temp15.txt [4] => temp22.txt )