Basic test case of what should occur:

<?php

	class test {
		protected	$str;

		public function __construct($str) {
			$this->str	= $str;
		}
	}


	$window		= new GtkWindow();
	$window->set_size_request(400, 400);


	// Tree Model
	$treeModel	= new GtkTreeStore(GObject::TYPE_STRING);
	$treeModel->append(null, array('foo'));

	// Tree View
	$treeView	= new GtkTreeView();
	$treeView->set_model($treeModel);
	$window->add($treeView);

	// Combo Model
	$comboModel	= new GtkListStore(GObject::TYPE_STRING, GObject::TYPE_PHP_VALUE);
	$comboModel->append(array('foo', new test('foo')));
	$comboModel->append(array('bar', new test('bar')));
	$comboModel->append(array('baz', new test('baz')));

	// Combo renderer
	$renderer			= new GtkCellRendererCombo();
	$renderer->set_property('model', $comboModel);
	$renderer->set_property('text-column', 0);
	$renderer->set_property('editable', true);
	$renderer->set_property('has-entry', false);
	$column				= new GtkTreeViewColumn('column:', $renderer, 'text', 0);
	$treeView->append_column($column);

    $renderer->connect('edited', 'combo_change');

	function combo_change(GtkCellRendererText $cell, $path, $selectionText/*, GtkTreeIter $treeIter*/) {
/*
		$class	= $cell->model->get_value($treeIter->iter, 1);
*/
		echo 'changed' . "\n";
	}

	$window->show_all();
	Gtk::main();

You're using the 'edited' signal - not the 'changed' signal. For the 'changed' 
signal, the GtkTreeIter is the third argument passed to the callback.