请看这份深度优先搜索代码，为何用引用就找不到？

Question

function search(&$node, &$forest, $id) { $node = null; $len = count($forest); for ($i=0; $i<$len; ++$i) { if ($forest[$i]['id'] == $id) { $node = &$forest[$i]; //【去掉“&”就能找到，加上就找不到】 return; } search($node, $forest[$i]['children'], $id); if ($node) { return; } } } $forest = array( array( 'id' => 1, 'pid' => 0, 'children' => array( array( 'id' => 11, 'pid' => 1, 'children' => array(), ), ), ), array( 'id' => 2, 'pid' => 0, 'children' => array(), ), ); search($node, $forest, 11); echo json_encode($node);