Something I have often needed to do in PHP is create hierarchical structures from an array. Usually I do this with recursive functions, but today I thought I would try something different - using references.

I needed to do it for a list of directories containing images, the hierarchical structure was created to resemble the folder structure the folders came from - folders with sub-folders.

function getFolderHierarchy($folders) {
  $hierarchy = array();
  $parents = array();
  $p = 0;
  $current =& $hierarchy;
  $depth = 1;
  $previous = null;

  foreach ($folders as $folder) {
    $path = explode(DIRECTORY_SEPARATOR, $folder['dir']);
    $pCount = count($path);

    if ($pCount > $depth) { // Sub-directory
      $parents[$p++] =& $current;
      $current =& $previous;
      $depth = $pCount;
    } else if ($pCount < $depth) {
      $current =&amp; $parents[--$p];
      $depth = $pCount;
    }

    $current[$path[$pCount-1]] = array(
      null => $folder['id']
    );
    $previous =&amp; $current[$path[$pCount-1]];
  }  

  return $hierarchy;
}

$folders = array(
  array(
    'dir' => 'South America',
    'id' => '1'
  ),
  array(
    'dir' => 'South America/Ecuador',
    'id' => '4'
  ),
  array(
    'dir' => 'South America/Ecuador/Alausi',
    'id' => '29'
  ),
  array(
    'dir' => 'South America/Ecuador/Alausi/Nariz del Diablo',
    'id' => '30'
  ),
  array(
    'dir' => 'South America/Ecuador/Amazon',
    'id' => '31'
  ),
  array(
    'dir' => 'South America/Ecuador/Banos',
    'id' => '10'
  ),
  array(
    'dir' => 'South America/Ecuador/Banos/Canyoning',
    'id' => '11'
  ),
  array(
    'dir' => 'South America/Ecuador/Cotopaxi National Park',
    'id' => '16'
  ),
  array(
    'dir' => 'South America/Ecuador/Cotopaxi National Park/Cotopaxi',
    'id' => '19'
  ),
  array(
    'dir' => 'South America/Ecuador/Cotopaxi National Park/Ruminahui',
    'id' => '18'
  ),
  array(
    'dir' => 'South America/Ecuador/Cotopaxi National Park/Secret Garden Cotopaxi',
    'id' => '17'
  ),
  array(
    'dir' => 'South America/Ecuador/Cuenca',
    'id' => '21'
  ),
  array(
    'dir' => 'South America/Ecuador/Cuenca/Cajas National Park',
    'id' => '23'
  ),
  array(
    'dir' => 'South America/Ecuador/Cuenca/Ingapirca',
    'id' => '22'
  ),
  array(
    'dir' => 'South America/Ecuador/Mindo',
    'id' => '24'
  ),
  array(
    'dir' => 'South America/Ecuador/Mindo/El Descanso',
    'id' => '26'
  ),
  array(
    'dir' => 'South America/Ecuador/Mindo/El Quetzal',
    'id' => '27'
  ),
  array(
    'dir' => 'South America/Ecuador/Mindo/Mariposas de Mindo',
    'id' => '28'
  ),
  array(
    'dir' => 'South America/Ecuador/Mindo/Tarabita',
    'id' => '25'
  ),
);

print_r(getFolderHierarchy($folders));

The following is returned when you run the above code

Array
(
  [South America] => Array
    (
      [] => 1
      [Ecuador] => Array
        (
          [] => 4
          [Alausi] => Array
            (
              [] => 29
              [Nariz del Diablo] => Array
                (
                  [] => 30
                )

            )

          [Amazon] => Array
            (
              [] => 31
            )

          [Banos] => Array
            (
              [] => 10
              [Canyoning] => Array
                (
                  [] => 11
                )

            )

          [Cotopaxi National Park] => Array
            (
              [] => 16
              [Cotopaxi] => Array
                (
                  [] => 19
                )

              [Ruminahui] => Array
                (
                  [] => 18
                )

              [Secret Garden Cotopaxi] => Array
                (
                  [] => 17
                )

            )

          [Cuenca] => Array
            (
              [] => 21
              [Cajas National Park] => Array
                (
                  [] => 23
                )

              [Ingapirca] => Array
                (
                  [] => 22
                )

            )

          [Mindo] => Array
            (
              [] => 24
              [El Descanso] => Array
                (
                  [] => 26
                )

              [El Quetzal] => Array
                (
                  [] => 27
                )

              [Mariposas de Mindo] => Array
                (
                  [] => 28
                )

              [Tarabita] => Array
                (
                  [] => 25
                )

            )

        )

    )

)