Sets a GtkTree as the subtree (or tree of children) for a node. A GtkTreeItem may only have one subtree. Attempting to set a second tree will generate a Gtk-Warning. The orginal subtree will not be overwritten or removed.

A node that has a subtree will be shown with a box to the left containing either a plus for a collapsed tree or a minus for an expanded tree.

<?php
// Create the top level tree item.
$tree     =& new GtkTree();
$treeItem =& new GtkTreeItem('Top Level Item');
$tree->append( $treeItem);

// Create the sub tree with one sub tree item.
$subTree     =& new GtkTree;
$subTreeItem =& new GtkTreeItem('Sub Level Item');

$subTree->append($subTreeItem);

// Add the sub tree to the top level item.
$treeItem->set_subtree($subTree);
?>

Controlling whether or not the subtree is visible is done using expand() and collapse() functions.