PHP序列化不按预期工作

Question

这是一段代码。已经实现了前缀树（trie）

<?php
class KTrie implements Serializable
{
function __construct()
{
    $this->root = new KTrieNode('');
}

function addWord($word)
{
    $wordArray = str_split($word);
    $this->root->addSuffix($wordArray);
}

function hasWord($word)
{
    $wordArray = str_split($word);
    return $this->root->hasSuffix($wordArray);
}

function printStructure()
{
    $this->root->printStructure(0);
}

function printWords()
{
    $this->root->printWords('');
}

public function serialize() {
    return serialize($this->data);
}
public function unserialize($data) {
    $this->data = unserialize($data);
}

}

class KTrieNode
{
function __construct($s){ $this->value = $s; }

function addSuffix($suffixArray)
{
    if(!empty($suffixArray))
    {
        $firstChar = $suffixArray[0];
        $remnant = array_slice($suffixArray, 1);

        $childNode = $this->getChild($firstChar);
        if($childNode === FALSE)
        {
            $childNode = $this->addChild($firstChar);
        }
        $childNode->addSuffix($remnant);
    }
    else
    {
        $this->finishesWord = TRUE;
    }
}

function hasSuffix($suffixArray)
{
    if(!empty($suffixArray))
    {
        $firstChar = $suffixArray[0];

        $childNode = $this->getChild($firstChar);
        if($childNode == FALSE)
        {
            return FALSE;
        }   
        else
        {
            $remnant = array_slice($suffixArray, 1);
            return $childNode->hasSuffix($remnant);
        }
    }
    else 
    {
        return TRUE;
    }
}       

function getChild($childString)
{
    if(is_array($this->children))
    {
        foreach ($this->children as $child)
        {
            if($child->value === $childString)
            {
                return $child;
            }
        }
    }
    return FALSE;
}

function addChild($childString)
{
    if(is_array($this->children))
    {
        foreach($this->children as $child)
        {
            if($child->value === $childString)
            {
                return $child;
            }
        }
    }

    $child = new KTrieNode($childString);
    $this->children[] = $child;

    return $child;
}

function printStructure($level)
{
    for($i=0; $i<$level; $i++)
    {
        echo ".";
    }
    echo $this->value.'<br/>';
    if(is_array($this->children))
    {
        foreach($this->children as $child)
        {
            $child->printStructure($level + 1);
        }
    }
}

function printWords($prefix)
{
    if($this->finishesWord)
    {
        echo $prefix.$this->value.'<br/>';
    }

    if(is_array($this->children))
    {
        foreach($this->children as $child)
        {
            $child->printWords($prefix.$this->value);
        }
    }
}
}


$trie = new KTrie();




$trie->addWord('123123123123');   // key 1
$trie->addWord('123123455644');  // key 2
$trie->addWord('12312354443');  // key 3
$trie->addWord('123123547787');  // key 3
/* and so on , for every item you add */

//$trie->printWords();

$ser=serialize($trie);


 $newobj =  unserialize($ser);

 var_dump($newobj->printWords());

?> 

使用var_dump($newobj->printWords());时出现的错误是 在非对象上调用成员函数printWords（）

为什么？因为在反序列化变量$ser后我应该得到类Ktrie的一个对象，因此函数printWords()理想情况下应该正常运行

非常感谢任何帮助

谢谢！