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.
    up
    24
    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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