php单例问题

A.php

<?php  
class Singleton  
{  
    private static $instance;  

        private function __construct()  
    {  
        echo "This is ok!\r\n";
    }  

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

B.php

<?php
require_once('A.php');
class AsyncOperation extends Thread {
  public function __construct($arg){
    $this->arg = $arg;
  }

  public function run(){
    if($this->arg){
      Singleton::GetInstance();
    }
  }
}


$arr = array();
for($i=0;$i<10;$i++){
    $arr[] = new AsyncOperation($i);
}

for($i=0;$i<10;$i++){
    $arr[$i]->start();
}

本人写java的 看到php的单例如A.php表示，但是实际在pthreads扩展下多线程跑的时候，会打印出内容，多次构造该实例。求各位phper解答疑问

对java熟悉点，java中

public class Singleton{
    private static Object instance = null;

    public Object getInstance(){
        if(instance==null){
            synchronized(Singleton.class){
                if(instance==null){
                    instance = new Singleton();
                }
            }
        }
        return instance
    }
}