请教这段 PHP 代码中，汉字的排序规则，

<?php
$fruits = array("张三", "orange","banana","apple" ,'dog','dophin',"王强",'王八','土豆','番茄','小米','红薯','兔子','大象');
sort($fruits);
$fruitslength = count($fruits);

foreach ($fruits as $key => $val) {

echo "fruit [".$key."] = ".$val." \n ";

}

echo $fruitslength;


?>

为什么结果是：

fruit [0] = apple
fruit [1] = banana
fruit [2] = dog
fruit [3] = dophin
fruit [4] = orange
fruit [5] = 兔子
fruit [6] = 土豆
fruit [7] = 大象
fruit [8] = 小米
fruit [9] = 张三
fruit [10] = 王八
fruit [11] = 王强
fruit [12] = 番茄
fruit [13] = 红薯
14

请问：兔子 土豆 大象 小米张三这些汉字是按照什么规则排序的？