帮忙看下程序，为什么显示不正确

<?php
class corporate_done
{
private $employeeid;
private $tiecolor;//define a setter and getter for $employeeid
function setemployeeid($employeeid){
$this->employeeid=$employeeid;
}
function getemployeeid(){
return $this->employeeid;
}//define a setter and getter for $tiecolor
function settiecolor($tiecolor){
$this->tiecolor=$tiecolor;
}
function gettiecolor(){
return $this->tiecolor;
}
}
function _clone(){
$this->tiecolor="blue";
}//create a new corporate_done object
$drone1=new corporate_done();
//set the $done1 employeeid number
$drone1->setemployeeid("12345");
//set the $drone1 tiecolor member
$drone1->settiecolor("red");
//clone the $drone1 object
$drone2=clone $drone1;
//set the $drone2 employeeid member
$drone2->setemployeeid("67890");
printf("drone1 employeeid:%d<br/>",$drone1->getemployeeid());
printf("drone1 tiecolor:%s<br/>",$drone1->gettiecolor());
printf("drone2 employeeid:%d<br/>",$drone2->getemployeeid());
printf("drone1 tiecolor:%s<br/>",$drone2->gettiecolor());
?>
我用的$drone2克隆$drone1,中途改变了颜色显示为blue，可是无论怎么弄，结果都显示red