PHP Cookbook

4.16. Sorting an Array

Problem

You want to sort an array in a specific way.

Solution

To sort an array using the traditional definition of sort, use sort( ) :

$states = array('Delaware', 'Pennsylvania', 'New Jersey');
sort($states);

To sort numerically, pass SORT_NUMERIC as the second argument to sort( ).

$scores = array(1, 10, 2, 20);
sort($scores, SORT_NUMERIC);

This resorts the numbers in ascending order (1, 2, 10, 20) instead of lexicographical order (1, 10, 2, 20).

Discussion

The sort( ) function doesn’t preserve the key/value association between elements; instead, entries are reindexed starting at 0 and going upward. (The one exception to this rule is a one-element array; its lone element doesn’t have its index reset to 0. This is fixed as of PHP 4.2.3.)

To preserve the key/value links, use asort( ). The asort( ) function is normally used for associative arrays, but it can also be useful when the indexes of the entries are meaningful:

$states = array(1 => 'Delaware', 'Pennsylvania', 'New Jersey');
asort($states);

while (list($rank, $state) = each($states)) {
    print "$state was the #$rank state to join the United States\n";

}

Use natsort( ) to sort the array using a natural sorting algorithm. Under natural sorting, you can mix strings and numbers inside your elements and still get the right answer.

$tests = array('test1.php', 'test10.php', 'test11.php', 'test2.php');
natsort($tests);

The elements are now ordered: 'test1.php', 'test2.php', 'test10.php', and 'test11.php'. With natural sorting, the number 10 comes after the number 2; the opposite occurs under traditional sorting. For case-insensitive natural sorting, use natcasesort( ).

To sort the array in reverse order, use rsort( ) or arsort( ), which is like rsort( ) but also preserves keys. There is no natrsort( ) or natcasersort( ). You can also pass SORT_NUMERIC into these functions.

See Also

Recipe 4.18 for sorting with a custom comparison function and Recipe 4.19 for sorting multiple arrays; documentation on sort( ) at http://www.php.net/sort, asort( ) at http://www.php.net/asort, natsort( ) at http://www.php.net/natsort, natcasesort( ) at http://www.php.net/natcasesort, rsort( ) at http://www.php.net/rsort, and arsort( ) at http://www.php.net/arsort.