How does the single-threaded programming language PHP implement multi-threaded operations?

<?php
/**
 * @title:        PHP多线程类(Thread)
 * @version:    1.0
 * @author:        php.cn < web@php.cn >
 * @published:    2010-11-2
 * 
 * PHP多线程应用示例：
 *  require_once &#39;thread.class.php&#39;;
 *  $thread = new thread();
 *  $thread->addthread(&#39;action_log&#39;,&#39;a&#39;);
 *  $thread->addthread(&#39;action_log&#39;,&#39;b&#39;);
 *  $thread->addthread(&#39;action_log&#39;,&#39;c&#39;);
 *  $thread->runthread();
 *  
 *  function action_log($info) {
 *      $log = &#39;log/&#39; . microtime() . &#39;.log&#39;;
 *      $txt = $info . "\r\n\r\n" . &#39;Set in &#39; . Date(&#39;h:i:s&#39;, time()) . (double)microtime() . "\r\n";
 *      $fp = fopen($log, &#39;w&#39;);
 *      fwrite($fp, $txt);
 *      fclose($fp);
 *  }
 */
class thread {
    var $hooks = array();
    var $args = array();
    function thread() {
    }
    function addthread($func)
    {
        $args = array_slice(func_get_args(), 1);
        $this->hooks[] = $func;
        $this->args[] = $args;
        return true;
    }
    function runthread()
    {
        if(isset($_GET[&#39;flag&#39;]))
        {
            $flag = intval($_GET[&#39;flag&#39;]);
        }
        if($flag || $flag === 0)
        {
            call_user_func_array($this->hooks[$flag], $this->args[$flag]);
        }
        else
        {
            for($i = 0, $size = count($this->hooks); $i < $size; $i++)
            {
                $fp=fsockopen($_SERVER[&#39;HTTP_HOST&#39;],$_SERVER[&#39;SERVER_PORT&#39;]);
                if($fp)
                {
                    $out = "GET {$_SERVER[&#39;PHP_SELF&#39;]}?flag=$i HTTP/1.1\r\n";
                    $out .= "Host: {$_SERVER[&#39;HTTP_HOST&#39;]}\r\n";
                    $out .= "Connection: Close\r\n\r\n";
                    fputs($fp,$out);
                    fclose($fp);
                }
            }
        }
    }
}
$thread = new thread();
$thread->addthread(&#39;func1&#39;,&#39;info1&#39;);
$thread->addthread(&#39;func2&#39;,&#39;info2&#39;);
$thread->addthread(&#39;func3&#39;,&#39;info3&#39;);
$thread->runthread();
//说明：
//addthread是添加线程函数，第一个参数是函数名，之后的参数（可选）为传递给指定函数的参数。
//runthread是执行线程的函数。