Make a Threaded multi level array from a two level array with parent id's. For example comments or a page tree. The class returns an array. Original code from http://www.jongales.com/blog/2009/01/27/php-class-for-threaded-comments/

Threaded.php

<?php

// From http://www.jongales.com/blog/2009/01/27/php-class-for-threaded-comments/
// This modified code now returns an array.

class Threaded {
	
	public $parents  = array();
	public $children = array();
	
	// @param array $threaded_data
	function __construct($threaded_data) {
		// Each nested array of the $data array needs to have an id and a parent_id key.
		foreach ($threaded_data as $data) {
			if ($data['parent_id'] === NULL) {
				$this->parents[$data['id']][] = $data;
			} else {
				$this->children[$data['parent_id']][] = $data;
			}
		}		
	}

	// @param array $data  Single data array
	// @param int $depth
	private function singleThread($data, $depth = 0) {
		foreach ($data as $k => $v) {
			$data[$k]['depth'] = $depth;

			if (isset($this->children[$data[$k]['id']])) {
			   $data[$k]['children'] = $this->singleThread($this->children[$data[$k]['id']], $depth + 1);
			}
		}
		return $data;
	}

	public function make() {
		$return = array();
		foreach ($this->parents as $c) {
			$single_thread = $this->singleThread($c);
			$return[] = $single_thread[0];
		}
		return $return;
	}
  
}



// EXAMPLE

$comments = array(
	array('id'=>1, 'parent_id'=>NULL, 'text'=>'Parent'),
	array('id'=>2, 'parent_id'=>1,	  'text'=>'Child'),
	array('id'=>3, 'parent_id'=>2,	  'text'=>'Child Third level'),
	array('id'=>4, 'parent_id'=>NULL, 'text'=>'Second Parent'),
	array('id'=>5, 'parent_id'=>4,    'text'=>'Second Child'),
	array('id'=>6, 'parent_id'=>NULL, 'text'=>'Parent 2'),
	array('id'=>7, 'parent_id'=>1,	  'text'=>'Child 2'),
	array('id'=>8, 'parent_id'=>2,	  'text'=>'Child Third level 2'),
	array('id'=>9, 'parent_id'=>NULL, 'text'=>'Second Parent 2'),
	array('id'=>10,'parent_id'=>4,    'text'=>'Second Child 2')
);

$t = new Threaded($comments);

?>

<pre><?php print_r( $t->make() ); ?></pre>