写一个PHP单例模式

<?php
/**
 * Created by PhpStorm. 5  * Date: 2019/1/29
 * Time: 17:44
 */

namespace App\Models;

use DfaFilter\SensitiveHelper;
use App\Models\Sensitive;

class StringHelper {

    static private $instance;
    static private $handle;

    //敏感词库不能为空，所以必须要有默认值
    private function getWordArray(){
        $sensitive = new Sensitive;
        $wordData = $sensitive ->getAllWordsCache();
        return $wordData;        
    }

    private function __construct(){
        self::$handle = SensitiveHelper::init()->setTree($this->getWordArray());
    }

    static public function getInstance(){
        if(!self::$instance instanceof self){
            self::$instance = new self();
        }
        return self::$instance;
    }

    //敏感词用*代替
    public function replace($content){
        if(empty($content)){
            return $content;
        }
        $content = self::$handle->replace($content,"***");
        return $content;
    }

    //判断是否包含敏感词 
    public function islegal($content){
        return self::$handle->islegal($content);
    }

    


}