函数的作用是：对数组按照键名逆向排序，元素的键名保持不变。
The krsort() function sorts an array by the keys in reverse order. The values keep their original keys.
当你对数组使用选择排序，你选择最小的元素，其余的阵列每一个周期的循环。
When you sort an array using selection sort, you choose the smallest element in the rest of the array on each cycle of the loop.
通过给定的对比方法把数组里面的元素按照升序排序。
Sorts the array's elements in ascending order, as determined by the comparison method specified by a given selector.
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