PHP infinite classification of tree class code

<?php 
 
class Tree 
{ 
var $data = array(); 
var $child = array(-1=>array()); 
var $layer = array(-1=>-1); 
var $parent = array(); 

function Tree ($value) 
{ 
$this->setNode(0, -1, $value); 
} // end func 

function setNode ($id, $parent, $value) 
{ 
$parent = $parent?$parent:0; 

$this->data[$id] = $value; 
$this->child[$id] = array(); 
$this->child[$parent][] = $id; 
$this->parent[$id] = $parent; 

if (!isset($this->layer[$parent])) 
{ 
$this->layer[$id] = 0; 
} 
else 
{ 
$this->layer[$id] = $this->layer[$parent] + 1; 
} 
} // end func 

function getList (&$tree, $root= 0) 
{ 
foreach ($this->child[$root] as $key=>$id) 
{ 
$tree[] = $id; 

if ($this->child[$id]) $this->getList($tree, $id); 
} 
} // end func 


function getValue ($id) 
{ 
return $this->data[$id]; 
} // end func 


function getLayer ($id, $space = false) 
{ 
return $space?str_repeat($space, $this->layer[$id]):$this->layer[$id]; 
} // end func 

function getParent ($id) 
{ 
return $this->parent[$id]; 
} // end func 

function getParents ($id) 
{ 
while ($this->parent[$id] != -1) 
{ 
$id = $parent[$this->layer[$id]] = $this->parent[$id]; 
} 

ksort($parent); 
reset($parent); 

return $parent; 
} // end func 

function getChild ($id) 
{ 
return $this->child[$id]; 
} // end func 

function getChilds ($id = 0) 
{ 
$child = array($id); 
$this->getList($child, $id); 

return $child; 
} // end func 
} // end class 


//New Tree(the name of the root directory);
//The ID of the root directory is automatically assigned to 0
$Tree = new Tree(' Directory navigation '); 

//SetNode (directory ID, parent ID, directory name);
$Tree->setNode(1, 0, ' directory 1'); 
$Tree->setNode(2, 1, ' directory 2'); 
$Tree->setNode(3, 0, ' directory 3'); 
$Tree->setNode(4, 3, ' directory 3.1'); 
$Tree->setNode(5, 3, ' directory 3.2'); 
$Tree->setNode(6, 3, ' directory 3.3'); 
$Tree->setNode(7, 2, ' directory 2.1'); 
$Tree->setNode(8, 2, ' directory 2.2'); 
$Tree->setNode(9, 2, ' directory 2.3'); 
$Tree->setNode(10, 6, ' directory 3.3.1'); 
$Tree->setNode(11, 6, ' directory 3.3.2'); 
$Tree->setNode(12, 6, ' directory 3.3.3'); 

//GetChilds (specify directory ID);
//Gets a subdirectory of the specified directory. If no directory is specified, start at the root directory
$category = $Tree->getChilds(); 

//Traverse the output
foreach ($category as $key=>$id) 
{ 
echo $Tree->getLayer($id, '|-').$Tree->getValue($id)."<br />n"; 
} 
?>