PHP natsort: How to Sort Arrays Using Natural Order

This article explains how to use the natsort() function in PHP to sort arrays using a natural order algorithm. You will learn the difference between standard alphanumeric sorting and natural sorting, see practical code examples demonstrating the function in action, and learn how to handle case-sensitivity during the sorting process.

What is Natural Order Sorting?

In standard computer sorting algorithms (like PHP’s default sort()), strings are compared character-by-character. This often leads to unnatural results when dealing with strings containing numbers. For example, a standard sort will place "img12.png" before "img2.png" because the character "1" comes before "2".

Natural order sorting orders strings the way a human naturally would. Using a natural order algorithm, "img2.png" is correctly sorted before "img12.png".

Using the natsort() Function

PHP provides the built-in natsort() function to achieve this. It sorts the elements of an array using natural order and maintains the original key-value associations.

Here is a basic example comparing sort() and natsort():

$files1 = ["img12.png", "img10.png", "img2.png", "img1.png"];
$files2 = $files1;

// Standard sorting
sort($files1);
echo "Standard sort:\n";
print_r($files1);

// Natural order sorting
natsort($files2);
echo "\nNatural order sort:\n";
print_r($files2);

Output:

Standard sort:
Array
(
    [0] => img1.png
    [1] => img10.png
    [2] => img12.png
    [3] => img2.png
)

Natural order sort:
Array
(
    [3] => img1.png
    [2] => img2.png
    [1] => img10.png
    [0] => img12.png
)

As shown in the output, natsort() correctly identifies that 2 is smaller than 10 and 12, and it preserves the original array keys (e.g., "img1.png" retains its original index of 3).

Case-Insensitive Natural Sorting

The natsort() function is case-sensitive, meaning uppercase letters will be sorted before lowercase letters. If you want to sort strings naturally without regard to case, you should use the natcasesort() function instead.

Here is an example demonstrating the difference:

$images = ["IMG2.png", "img10.png", "img1.png", "IMG12.png"];

// Case-sensitive natural sort
natsort($images);
echo "Case-sensitive (natsort):\n";
print_r($images);

// Case-insensitive natural sort
natcasesort($images);
echo "\nCase-insensitive (natcasesort):\n";
print_r($images);

Output:

Case-sensitive (natsort):
Array
(
    [0] => IMG2.png
    [3] => IMG12.png
    [2] => img1.png
    [1] => img10.png
)

Case-insensitive (natcasesort):
Array
(
    [2] => img1.png
    [0] => IMG2.png
    [1] => img10.png
    [3] => IMG12.png
)

Key Behavior to Remember

• Key Preservation: Both natsort() and natcasesort() maintain the key-value associations of the array. If you need to reset the keys to consecutive integers after sorting, use the array_values() function on the sorted array.
• In-place Sorting: These functions modify the original array directly and return true on success or false on failure, rather than returning a new sorted array.