Grouping Array values while maintaining order within group?

What is the best way to group and still maintain order with in groups?

I have an array full of directory paths to files. The array file paths are ordered by the files creation dates.

I want to keep the files in order of creation date but also group them in order of what directory they are in.

Example:

array() {
[1]=> 开发者_运维技巧"media/abstract/safe.png"
[2]=> "media/urban/Cross Street 2.png"
[4]=> "media/urban/The Other Side.png"
[5]=> "media/urban/Pharm Child.png"
[6]=> "media/food/DSC_3017.png"
[7]=> "media/abstract/fractal_twins.png"
[9]=> "media/urban/Amsterdam.png"
[11]=> "media/nature/creep.png"
[12]=> "media/urban/obstructed.png"
[13]=> "media/nature/middletown_dreams.png"
}

reordered would look like:

array() {
[1]=> "media/abstract/safe.png"
[2]=> "media/abstract/fractal_twins.png"
[3]=> "media/food/DSC_3017.png"
[4]=> "media/nature/creep.png"
[5]=> "media/nature/middletown_dreams.png"
[6]=> "media/urban/Cross Street 2.png"
[7]=> "media/urban/The Other Side.png"
[8]=> "media/urban/Pharm Child.png"
[9]=> "media/urban/Amsterdam.png"
[10]=> "media/urban/obstructed.png"
}

They are now grouped by their directories, but within their directories they maintain their order relative to one another.

It's not quite straightforward because all of the PHP sorting functions sort by only key or value (as far as I know). But the information needed is in both the key and value. So you'll need to modify the array first, then sort, then change it back to just have the file names.

function sort_files(array &$files)
{
  /* add extended info:
      0: index (original order)
      1: dir name
      2: original file name */
  $i = 0;
  $files = array_map(function($name) use (&$i) {
    return array($i++, dirname($name), $name);
  }, $files);

  /* sort extended array by dir then index */
  usort($files, function($a, $b) {
    $cmp = strcmp($a[1], $b[1]);
    if (!$cmp) $cmp = $a[0] - $b[0];
    return $cmp;
  });

  /* remove extended info from array */
  $files = array_map(function($a) {
    return $a[2];
  }, $files);
}

sort_files($files);

Output:

Array
(
    [0] => media/abstract/safe.png
    [1] => media/abstract/fractal_twins.png
    [2] => media/food/DSC_3017.png
    [3] => media/nature/creep.png
    [4] => media/nature/middletown_dreams.png
    [5] => media/urban/Cross Street 2.png
    [6] => media/urban/The Other Side.png
    [7] => media/urban/Pharm Child.png
    [8] => media/urban/Amsterdam.png
    [9] => media/urban/obstructed.png
)