This function implements a sort algorithm that orders alphanumeric strings in the way a human being would. This is described as a "natural ordering". An example of the difference between this algorithm and the regular computer string sorting algorithms (used in sort()) can be seen below:
The code above will generate the following output:
Standard sorting Array (  => img1.png  => img10.png  => img12.png  => img2.png ) Natural order sorting Array (  => img1.png  => img2.png  => img10.png  => img12.png )
Note: If you're wanting to maintain index/value associations, consider using usort($arr, 'strnatcmp').