Use krsort to solve the problem of repeated sorting of array keys in PHP

gitbox 2025-05-26

In PHP, when sorting arrays, especially associative arrays (key-value pair arrays), we often encounter duplicate key names or the sorting does not meet expectations. krsort is a function in PHP for reverse sorting by key names. Can it solve the problem of "key duplication leads to sorting"? This article will analyze in detail and give specific operation examples.

1. Uniqueness of PHP array key names

First of all, it should be clear that the key names of PHP arrays are unique. If you use a duplicate key name when assigning values to an array, the value assigned afterwards will overwrite the previous one. For example:

 <?php
$arr = [
    "key1" => "value1",
    "key2" => "value2",
    "key1" => "value3"
];
print_r($arr);
?>

The output result is:

 Array
(
    [key1] => value3
    [key2] => value2
)

It can be seen that when key1 is repeated, value1 is overwritten by value3 , and in fact only the last assignment is retained in the array.

Therefore, it is impossible to have duplicate key names in PHP arrays , which means that if you feel that there is a problem with duplicate key names, it is likely that you have a wrong understanding of the array structure or you are using a multidimensional array or some kind of nested structure.

2. The role of krsort

The krsort function is used to reverse sort (descending) according to the key names of the array, preserving the key-value relationship. The function definition is as follows:

 bool krsort ( array &$array [, int $sort_flags = SORT_REGULAR ] )

Example:

 <?php
$arr = [
    "b" => 2,
    "a" => 1,
    "c" => 3
];

krsort($arr);
print_r($arr);
?>

Output result:

 Array
(
    [c] => 3
    [b] => 2
    [a] => 1
)

Note that krsort does not affect the uniqueness of the keys , it simply reverses the existing keys.

3. How to solve the problem of "duplicate" of key names?

Because of the unique feature of PHP array key names, the so-called "sorting problems caused by key duplication" may refer to:

Key names at different levels in multidimensional arrays are repeated, but unique in the same level.
You want to sort by a field, but the key name is not unique, or the data structure is unreasonable.

3.1 Solution Example 1: Reorganize Array Keys

If you have a multidimensional array like the following:

 <?php
$arr = [
    ["id" => 1, "name" => "Alice"],
    ["id" => 2, "name" => "Bob"],
    ["id" => 1, "name" => "Charlie"]
];

There are duplicates for "id" here, but it is a value in the array, not a key name. You want to sort the "id" field:

 <?php
// according toidSort in reverse order
usort($arr, function($a, $b){
    return $b['id'] <=> $a['id'];
});

print_r($arr);
?>

This has nothing to do with krsort , which is only sorted for key names.

3.2 Solution Example 2: Reconstruct an array using a unique key

If you really need to use a field as a key to avoid duplication of key names, you can filter or splice the key names first:

 <?php
$arr = [
    ["id" => 1, "name" => "Alice"],
    ["id" => 2, "name" => "Bob"],
    ["id" => 1, "name" => "Charlie"]
];

// byidandnameCombination as a unique key
$newArr = [];
foreach ($arr as $item) {
    $key = $item['id'] . '_' . $item['name']; // Guaranteed key unique
    $newArr[$key] = $item;
}

// 对键Sort in reverse order
krsort($newArr);

print_r($newArr);
?>

In this way, you can not only avoid duplication of key names, but also sort them with krsort .

4. Summary

The key names in the PHP array must be unique, and there is no real "key name duplication".
krsort is used to reverse sort by key names and will not solve the problem of duplication of key names.
If you have "repeated keys" in your data, it is often a problem with the data structure, and the array structure should be redesigned.
For sorting by a certain field, custom sorting methods such as usort should be used.
If you want to sort by key names and make sure that the key names are unique, you can splice the fields to ensure that they are unique, and then use krsort .

If you are interested in array key sorting, recommend the official PHP document: krsort — Sort an array by key in reverse order

krsort
usort