如何解决PHP无法实现多线程的问题

有没有办法在PHP中实现多线程呢？假设你正在写一个基于多台服务器的PHP应用，理想的情况时同时向多台服务器发送请求，而不是一台接一台。可以实现吗？当有人想要实现并发功能时，他们通常会想到用fork或者spawn threads，但是当他们发现PHP不支持多线程的时候，大概会转换思路去用一些不够好的语言，比如Perl。
假设你要建立一个服务来检查正在运行的n台服务器，以确定他们还在正常运转。你可能会写下面这样的代码：

$hosts = array("host1.sample.com", "host2.sample.com", "host3.sample.com"); $timeout = 15; $status = array(); foreach ($hosts as $host) {      $errno = 0;      $errstr = "";      $s = fsockopen($host, 80, $errno, $errstr, $timeout);      if ($s) {         $status[$host] = "Connectedn";         fwrite($s, "HEAD / HTTP/1.0rnHost: $hostrnrn");        do {           $data = fread($s, 8192);           if (strlen($data) == 0) {           break;           }          $status[$host] .= $data;       }       while (true);        fclose($s);       }       else {         $status[$host] = "Connection failed: $errno $errstrn";       } } print_r($status); ?>

它运行的很好，但是在fsockopen()分析完hostname并且建立一个成功的连接（或者延时$timeout秒）之前，扩充这段代码来管理大量服务器将耗费很长时间。

因此我们必须放弃这段代码；我们可以建立异步连接-不需要等待fsockopen返回连接状态。PHP仍然需要解析hostname（所以直接使用ip更加明智），不过将在打开一个连接之后立刻返回，继而我们就可以连接下一台服务器。

有两种方法可以实现；PHP5中可以使用新增的stream_socket_client()函数直接替换掉fsocketopen()。PHP5之前的版本，你需要自己动手，用sockets扩展解决问题。下面是PHP5中的解决方法：

$hosts = array("host1.sample.com", "host2.sample.com", "host3.sample.com"); $timeout = 15; $status = array(); $sockets = array(); /* Initiate connections to all the hosts simultaneously */ foreach ($hosts as $id => $host) {      $s = stream_socket_client("$host:80", $errno, $errstr, $timeout,         STREAM_CLIENT_ASYNC_CONNECT|STREAM_CLIENT_CONNECT);      if ($s) {        $sockets[$id] = $s;        $status[$id] = "in progress";      }      else { $status[$id] = "failed, $errno $errstr";      } } /* Now, wait for the results to come back in */  while (count($sockets)) {     $read = $write = $sockets;  /* This is the magic function - explained below */     $n = stream_select($read, $write, $e = null, $timeout);     if ($n > 0) {     /* readable sockets either have data for us, or are failed  * connection attempts */       foreach ($read as $r) {              $id = array_search($r, $sockets);              $data = fread($r, 8192);         if (strlen($data) == 0) {             if ($status[$id] == "in progress") {               $status[$id] = "failed to connect";             }        fclose($r);        unset($sockets[$id]);          }        else {            $status[$id] .= $data;          }      }  /* writeable sockets can accept an HTTP request */  foreach ($write as $w) {        $id = array_search($w, $sockets);        fwrite($w, "HEAD / HTTP/1.0rnHost: "        . $hosts[$id] . "rnrn");        $status[$id] = "waiting for response";       }  }  else {  /* timed out waiting; assume that all hosts associated  * with $sockets are faulty */  foreach ($sockets as $id => $s) {        $status[$id] = "timed out "       . $status[$id];       }  break;   } } foreach ($hosts as $id => $host) {     echo "Host: $hostn"; echo "Status: "     . $status[$id] . "nn"; }  ?>