class async_file_get_contents extends Thread{
    public $ret;
    public $url;
    public $finished;
        public function __construct($url) {
        $this->finished=false;
        $this->url=$url;
    }
        public function run() {
        $this->ret=file_get_contents($this->url);
        $this->finished=true;
    }
}
$afgc=new async_file_get_contents("http://example.org/file.ext");

您可以使用exec()来调用一些可以执行HTTP请求的东西，如wget，但必须将程序的所有输出指向某个地方，如文件或/dev/null，否则PHP进程将等待该输出。

如果你想把进程和apache线程完全分开，可以尝试这样做(我不确定，但我希望你能明白):

exec('bash -c "wget -O (url goes here) > /dev/null 2>&1 &"');

这不是一项很好的业务，您可能需要类似cron作业的东西来调用heartbeat脚本，该脚本轮询实际的数据库事件队列来执行真正的异步事件。

截至2018年，Guzzle已经成为HTTP请求事实上的标准库，在几个现代框架中使用。它是用纯PHP编写的，不需要安装任何自定义扩展。

它可以很好地执行异步HTTP调用，甚至可以在需要执行100个HTTP调用，但不想一次运行超过5个的情况下将它们合并在一起。

并发请求示例

use GuzzleHttp\Client;
use GuzzleHttp\Promise;

$client = new Client(['base_uri' => 'http://httpbin.org/']);

// Initiate each request but do not block
$promises = [
    'image' => $client->getAsync('/image'),
    'png'   => $client->getAsync('/image/png'),
    'jpeg'  => $client->getAsync('/image/jpeg'),
    'webp'  => $client->getAsync('/image/webp')
];

// Wait on all of the requests to complete. Throws a ConnectException
// if any of the requests fail
$results = Promise\unwrap($promises);

// Wait for the requests to complete, even if some of them fail
$results = Promise\settle($promises)->wait();

// You can access each result using the key provided to the unwrap
// function.
echo $results['image']['value']->getHeader('Content-Length')[0]
echo $results['png']['value']->getHeader('Content-Length')[0]

看到http://docs.guzzlephp.org/en/stable/quickstart.html并发请求

您可以使用这个库:https://github.com/stil/curl-easy

这很简单:

<?php
$request = new cURL\Request('http://yahoo.com/');
$request->getOptions()->set(CURLOPT_RETURNTRANSFER, true);

// Specify function to be called when your request is complete
$request->addListener('complete', function (cURL\Event $event) {
    $response = $event->response;
    $httpCode = $response->getInfo(CURLINFO_HTTP_CODE);
    $html = $response->getContent();
    echo "\nDone.\n";
});

// Loop below will run as long as request is processed
$timeStart = microtime(true);
while ($request->socketPerform()) {
    printf("Running time: %dms    \r", (microtime(true) - $timeStart)*1000);
    // Here you can do anything else, while your request is in progress
}

下面你可以看到上面例子的控制台输出。 它将显示简单的实时时钟，指示请求正在运行的时间:

这是我自己的PHP函数，当我做POST到任何页面的特定URL .... 示例:my Function的***用法…

    <?php
        parse_str("email=myemail@ehehehahaha.com&subject=this is just a test");
        $_POST['email']=$email;
        $_POST['subject']=$subject;
        echo HTTP_POST("http://example.com/mail.php",$_POST);***

    exit;
    ?>
    <?php
    /*********HTTP POST using FSOCKOPEN **************/
    // by ArbZ

function HTTP_Post($URL,$data, $referrer="") {

    // parsing the given URL
    $URL_Info=parse_url($URL);

    // Building referrer
    if($referrer=="") // if not given use this script as referrer
        $referrer=$_SERVER["SCRIPT_URI"];

    // making string from $data
    foreach($data as $key=>$value)
        $values[]="$key=".urlencode($value);
        $data_string=implode("&",$values);

    // Find out which port is needed - if not given use standard (=80)
    if(!isset($URL_Info["port"]))
        $URL_Info["port"]=80;

    // building POST-request: HTTP_HEADERs
    $request.="POST ".$URL_Info["path"]." HTTP/1.1\n";
    $request.="Host: ".$URL_Info["host"]."\n";
    $request.="Referer: $referer\n";
    $request.="Content-type: application/x-www-form-urlencoded\n";
    $request.="Content-length: ".strlen($data_string)."\n";
    $request.="Connection: close\n";
    $request.="\n";
    $request.=$data_string."\n";

    $fp = fsockopen($URL_Info["host"],$URL_Info["port"]);
    fputs($fp, $request);
    while(!feof($fp)) {
        $result .= fgets($fp, 128);
    }
    fclose($fp); //$eco = nl2br();


    function getTextBetweenTags($string, $tagname) {
        $pattern = "/<$tagname ?.*>(.*)<\/$tagname>/";
        preg_match($pattern, $string, $matches);
        return $matches[1];
    }
    //STORE THE FETCHED CONTENTS to a VARIABLE, because its way better and fast...
    $str = $result;
    $txt = getTextBetweenTags($str, "span"); $eco = $txt;  $result = explode("&",$result);
    return $result[1];
    <span style=background-color:LightYellow;color:blue>".trim($_GET['em'])."</span>
    </pre> "; 
}
</pre>

如果你控制了你想要异步调用的目标(例如你自己的"longtask.php")，你可以从那端关闭连接，两个脚本将并行运行。它是这样工作的:

quick.php通过cURL打开longtask.php(这里没有魔法) php关闭连接并继续(神奇!) 当连接关闭时，cURL返回quick.php 两项任务同时进行

我试过了，效果不错。但是quick.php不会知道longtask.php正在做什么，除非您在进程之间创建一些通信方式。

在执行其他操作之前，请先在longtask.php中尝试这段代码。它将关闭连接，但仍然继续运行(并抑制任何输出):

while(ob_get_level()) ob_end_clean();
header('Connection: close');
ignore_user_abort();
ob_start();
echo('Connection Closed');
$size = ob_get_length();
header("Content-Length: $size");
ob_end_flush();
flush();

代码复制自PHP手册的用户贡献注释，并进行了一些改进。