Sorting Arrays

PHP has several functions that deal with sorting arrays, and this document exists to help sort it all out.

The main differences are:

Some sort based on the array keys, whereas others by the values: $array['key'] = 'value';
Whether or not the correlation between the keys and values are maintained after the sort, which may mean the keys are reset numerically (0,1,2 ...)
The order of the sort: alphabetical, ascending (low to high), descending (high to low), natural, random, or user defined
Note: All of these sort functions act directly on the array variable itself, as opposed to returning a new sorted array
If any of these sort functions evaluates two members as equal then they retain their original order. Prior to PHP 8.0.0, their order were undefined (the sorting was not stable).

**Sorting function attributes**
Function name	Sorts by	Maintains key association	Order of sort	Related functions
array_multisort()	value	string keys yes, int keys no	first array or sort options	array_walk()
asort()	value	yes	ascending	arsort()
arsort()	value	yes	descending	asort()
krsort()	key	yes	descending	ksort()
ksort()	key	yes	ascending	krsort()
natcasesort()	value	yes	natural, case insensitive	natsort()
natsort()	value	yes	natural	natcasesort()
rsort()	value	no	descending	sort()
shuffle()	value	no	random	array_rand()
sort()	value	no	ascending	rsort()
uasort()	value	yes	user defined	uksort()
uksort()	key	yes	user defined	uasort()
usort()	value	no	user defined	uasort()

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User Contributed Notes 2 notes

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142

"Matthew Rice" ¶

11 years ago

While this may seem obvious, user-defined array sorting functions ( uksort(), uasort(), usort() ) will *not* be called if the array does not have *at least two values in it*.

The following code:                        

<?php

function usortTest($a, $b) {
    var_dump($a);
    var_dump($b);
    return -1;
}

$test = array('val1');
usort($test, "usortTest");

$test2 = array('val2', 'val3');
usort($test2, "usortTest");

?>

Will output: 

string(4) "val3"
string(4) "val2"

The first array doesn't get sent to the function.

Please, under no circumstance, place any logic that modifies values, or applies non-sorting business logic in these functions as they will not always be executed.

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oculiz at gmail dot com ¶

13 years ago

Another way to do a case case-insensitive sort by key would simply be:

<?php
uksort($array, 'strcasecmp');
?>

Since strcasecmp is already predefined in php it saves you the trouble to actually write the comparison function yourself.

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