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PHP Sorting Arrays

The simplest sorting functions provided by PHP are sort() and rsort() which sort arrays in ascending and descending order respectively.

The sort() and rsort Functions

The sort() function takes the array as a parameter and sorts numeric values in numeric order and string values in alphabetical order.

The rsort() function does the same but in descending order.

Example of sort()

<?php
    $numbers = array (10, 35, 2, 12);

    sort ($numbers);

    // Print out the result
    foreach ($numbers as $number)
        echo($number.', ');

    // Which gives 2, 10, 12, 35,

    $names = array ("Tom", "Dick", "Harry");

    sort ($names);

    // Print out the result
    foreach ($names as $name)
        echo($name.', ');

    // Which gives Dick, Harry, Tom,
?>

The way these functions sort can be changed by using the optional parameter sort_flag.

The syntax is sort ($array, sort_flag) where sort_flag is one of the values shown below.

SORT_REGULAR - compare items normally.
SORT_NUMERIC - compare items numerically.
SORT_STRING - compare items as strings.

Note: Although sort() and rsort() can be used on associative arrays, the keys will be lost.

Sorting Associative arrays

If you don't wish the keys to be lost when you sort an associative array then the asort() and arsort() functions are available.

These work in the same way as sort() and rsort() but the keys are preserved.

Example of asort()

<?php
    $names = array ("first" => "Tom", "second" => "Dick", "third" => "Harry");

    asort ($names);

    // Print out the result
    foreach ($names as $index => $name)
        echo('['.$index.']'.$name.' ');

    // Which gives [second]Dick [third]Harry [first]Tom
?>

Note: . Although asort() and arsort() can be used on numbered arrays, PHP treats the indexes as keys and maintains the association of the keys with the values of the elements.

This means that the indexes won't run in order as you might expect.

Sorting by Key

The ksort() and krsort() functions work in the same way as asort() and arsort() except that the array is sorted by key instead of by element value.

Example of ksort()

<?php
    $names = array ("first" => "Tom", "next" => "Dick", "last" => "Harry");

    ksort ($names);

    // Print out the result
    foreach ($names as $index => $name)
        echo('['.$index.']'.$name.' ');

    // Which gives [first]Tom [last]Harry [next]Dick
?>

User-Defined Sorting

This requires a knowledge of functions, which is covered in the tutorial User Defined Functions.

PHP provides three functions to allow user-defined sorting.

usort() which sorts the array based on the value of each element.
uasort() which does the same but preserves the association between the element value and key.
uksort() which sorts the array based on the key rather than the value of each element.

The syntax of all three functions takes the form
usort ($array, cmp_function)
where cmp_function is the name of a user-defined function that compares two arguments (either element values or keys) to decide if they are equal, one is greater than the other or one is less the other.

The user-defined function is not called directly from within the script but its name passed as a parameter to the sort function. Functions used in this way are normally known as callback functions.

The function can be used to make any comparison you choose. Below is an example that compares the length of two strings and returns 0 if they're equal, 1 if the first is greater than the second and -1 if the first is less than the second.

Example of a Callback Function

<?php
    function compare_names ($name1, $name2)
    {
        if (strlen ($name1) > strlen ($name2)) return 1;
        if (strlen ($name1) < strlen ($name2)) return -1;
        return 0;
    }
?>

The function above can now be used to sort an array of names by their length.

User-Defined Sort

<?php
    $names = array ("Tom", "Dick", "Harry", "Ed", "George");

    usort ($names, "compare_names");

    // Print out the result
    foreach ($names as $name)
        echo($name.' ');

    // Which gives Ed Tom Dick Harry George
?>

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