php 控制preg_replace替换次数

$v = array('vv','bb');
$str = "vv,cc,abcde,www.111cn.net,vv";
$str=preg_replace("/$v[0]/","<a href=".$v[1]." target=_blank >".$v[0]."</a>",$str,1);
echo $str;
//bb,cc,abcde,www.111cn.net,vv

$str=preg_replace("/$v[0]/","<a href=".$v[1]." target=_blank >".$v[0]."</a>",$str,2);
//bb,cc,abcde,www.111cn.net,bb

$url = "http://".$url.$_server["request_uri"];
header("http/1.1 301 moved permanently");
header ("location:http://$url");

function init()
{
  $ch = curl_init();
  curl_setopt ($ch, curlopt_url, $url);
  curl_setopt ($ch, curlopt_returntransfer, 1);
  curl_setopt ($ch, curlopt_connecttimeout, $timeout);
  $file_contents = curl_exec($ch);
  curl_close($ch);
}
//方法三最简单的用file_get_contents

function getfiles($value)
{
 $get_file = @file_get_contents($value);
}
//方法四用fopen采集远程网页内容

function getfiles($value)
{
 return fopen($value);
}

function mkdirs($dir)
{
 if(!is_dir($dir)){
  if(!mkdirs(dirname($dir))){
   return false;}
  if(!mkdir($dir,0777)){
   return false;}
 }
 return true;
}
//测试方法
$img_path = realpath("../../../upfile/www.111cn.net/").'/'.date("y/m/d/");
mkdirs($img_path);

//函数了
function mkdir_r($dirname, $rights=0777){
    $dirs = explode('/', $dirname);
    $dir='';
    foreach ($dirs as $part) {
        $dir.=$part.'/';
        if (!is_dir($dir) && strlen($dir)>0)
            mkdir($dir, $rights);
    }
}

$path ="www.111cn.net/".date("y/m");
mkdir_r($path);

class timer
{
var $starttime = 0;
var $stoptime = 0;
var $timespent = 0;

function start(){
$this->starttime = microtime();}
function stop(){
$this->stoptime　= microtime();}
function spent(){
if ($this->timespent) {
return $this->timespent;
} else {
$startmicro = substr($this->starttime,0,10);
$startsecond = substr($this->starttime,11,10);
$stopmicro　 = substr($this->stoptime,0,10);
$stops教程econd　= substr($this->stoptime,11,10);
$start = doubleval($startmicro) + $startsecond;
$stop　= doubleval($stopmicro) + $stops教程econd;
$this->timespent = $stop - $start;
return substr($this->timespent,0,8)."秒";
}
}// end function spent();
}//end class timer;
//例子
$timer = new timer;
$timer->start();
/*
你的代码放在此处
*/

$timer->stop();
echo "执行本script共".$timer->spent();