ksort()

PHP Function: Array ksort()

The PHP ksort() function sorts an array by its keys in ascending order, keeping each key associated with its original value. It is the key-based counterpart to asort(), which sorts by value, and the opposite of krsort(), which sorts keys in descending order.

This page covers when to reach for ksort(), its syntax, parameters and return value, the available sort-type flags, and several worked examples (including the common gotcha of sorting numeric string keys).

When to use ksort()

Use ksort() when the keys carry meaning and you want the array to be ordered by them — for example:

• Sorting an associative array of name => value pairs alphabetically by name.
• Putting configuration entries or HTTP headers into a predictable, deterministic order.
• Ordering data keyed by year, ID, or category before iterating or rendering it.

Because ksort() preserves the key/value association, it is the right tool when reindexing the array (as plain sort() would do) would lose information. See the Sorting Arrays overview for a comparison of all the PHP sort functions.

Syntax

ksort($array, $sorting_type);

Parameters

The ksort() function takes two parameters:

The $sorting_type flag accepts the following constants:

Note: ksort() modifies the original array in place — it does not return a new sorted array. The original key order is lost after the call.

Return Value

The ksort() function returns a boolean: true on success and false on failure.

Example 1: Sorting an Associative Array by Key

Here the keys are people's names, so ksort() orders the array alphabetically by name (Ben, Joe, Peter) while keeping each name paired with the correct age:

Output:

Array
(
    [Ben] => 37
    [Joe] => 43
    [Peter] => 35
)

Example 2: Sorting an Associative Array by Key in Reverse Order

For descending key order, use krsort() instead — it works exactly like ksort() but reverses the result:

Output:

Array
(
    [Peter] => 35
    [Joe] => 43
    [Ben] => 37
)

Example 3: Sorting an Indexed Array by Key

An indexed array already has keys 0, 1, 2, 3 in order, so sorting it by key leaves the elements unchanged. This shows that ksort() sorts keys, not values — to sort the values themselves, use sort():

Output:

Array
(
    [0] => red
    [1] => green
    [2] => blue
    [3] => yellow
)

Example 4: Sorting an Indexed Array by Key in Reverse Order

Reversing the key order of an indexed array with krsort() keeps each value with its original numeric key, so the elements appear in reverse insertion order:

Output:

Array
(
    [3] => yellow
    [2] => blue
    [1] => green
    [0] => red
)

Gotcha: numeric string keys and the sort flag

A common surprise is sorting an array whose keys are numeric strings. By default (SORT_REGULAR) PHP compares them as numbers, so "10" comes after "2". Passing SORT_STRING compares them character by character, putting "10" before "2":

<?php

$data = ["10" => "a", "1" => "b", "2" => "c"];

ksort($data);                // SORT_REGULAR (numeric)
print_r($data);

ksort($data, SORT_STRING);   // string comparison
print_r($data);

Output:

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

Choose the $sorting_type flag that matches how you want the keys interpreted.

Conclusion

The ksort() function sorts a PHP array by its keys in ascending order, in place, while preserving each key/value pair. Reach for it when the keys are meaningful and order matters; use krsort() for descending order, asort() / arsort() to sort by value while keeping keys, or sort() when you want to discard the keys entirely. Pick the right $sorting_type flag — especially when your keys are numeric strings — to get the comparison you expect.

graph TD
    A[PHP array] -->|ksort| B[Sorted PHP array]

Practice