Recursive function to generate multidimensional array from database result

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Some very simple, generic tree building:

function buildTree(array $elements, $parentId = 0) {
    $branch = array();

    foreach ($elements as $element) {
        if ($element['parent_id'] == $parentId) {
            $children = buildTree($elements, $element['id']);
            if ($children) {
                $element['children'] = $children;
            }
            $branch[] = $element;
        }
    }

    return $branch;
}

$tree = buildTree($rows);

The algorithm is pretty simple:

  1. Take the array of all elements and the id of the current parent (initially 0/nothing/null/whatever).
  2. Loop through all elements.
  3. If the parent_id of an element matches the current parent id you got in 1., the element is a child of the parent. Put it in your list of current children (here: $branch).
  4. Call the function recursively with the id of the element you have just identified in 3., i.e. find all children of that element, and add them as children element.
  5. Return your list of found children.

In other words, one execution of this function returns a list of elements which are children of the given parent id. Call it with buildTree($myArray, 1), it will return a list of elements which have the parent id 1. Initially this function is called with the parent id being 0, so elements without parent id are returned, which are root nodes. The function calls itself recursively to find children of children.

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