Explode tree function (taken from http://kvz.io/blog/2007/10/03/convert-anything-to-tree-structures-in-php/)

explode_tree.php

<?php
function explodeTree($array, $delimiter = "_", $baseval = false) {
	if(!is_array($array)) return false;
	$splitRE = "/" . preg_quote($delimiter, "/") . "/";
	$returnArr = array();
	foreach ($array as $key => $val) {
		// Get parent parts and the current leaf
		$parts = preg_split($splitRE, $key, -1, PREG_SPLIT_NO_EMPTY);
		$leafPart = array_pop($parts);

		// Build parent structure
		// Might be slow for really deep and large structures
		$parentArr = &$returnArr;
		foreach ($parts as $part) {
			if (!isset($parentArr[$part])) {
				$parentArr[$part] = array();
			} elseif (!is_array($parentArr[$part])) {
				if ($baseval) {
					$parentArr[$part] = array("__base_val" => $parentArr[$part]);
				} else {
					$parentArr[$part] = array();
				}
			}
			$parentArr = &$parentArr[$part];
		}

		// Add the final part to the structure
		if (empty($parentArr[$leafPart])) {
			$parentArr[$leafPart] = $val;
		} elseif ($baseval && is_array($parentArr[$leafPart])) {
			$parentArr[$leafPart]["__base_val"] = $val;
		}
	}
	return $returnArr;
}
?>

plot_tree.php

<?php
function plotTree($arr, $indent=0, $mother_run=true){
	if ($mother_run) {
		// the beginning of plotTree. We're at rootlevel
		print "start<br />";
	}

	foreach ($arr as $k=>$v){
		// skip the baseval thingy. Not a real node.
		if ($k == "__base_val") continue;
		// determine the real value of this node.
		$show_val = (is_array($v) ? $v["__base_val"] : $v);
		// show the indents
		print str_repeat("  ", $indent);
		if ($indent == 0) {
			// this is a root node. no parents
			print "O ";
		} elseif (is_array($v)){
			// this is a normal node. parents and children
			print "+ ";
		} else {
			// this is a leaf node. no children
			print "- ";
		}

		// show the actual node
		print $k . " (" . $show_val. ")" . "<br />";
		if (is_array($v)) {
			// this is what makes it recursive, rerun for childs
			plotTree($v, ($indent+1), false);
		}
	}

	if ($mother_run) {
		print "end<br />";
	}
}
?>