通过curl函数：

$post_data = array();
$post_data['clientname'] = "test08";
$post_data['clientpasswd'] = "test08";
$post_data['submit'] = "submit";
$url='http://xxx.xxx.xxx.xx/xx/xxx/top.php';
$o="";
foreach ($post_data as $k=>$v)
{
    $o.= "$k=".urlencode($v)."&";
}
$post_data=substr($o,0,-1);
$ch = curl_init();
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_HEADER, 0);
curl_setopt($ch, CURLOPT_URL,$url);
//为了支持cookie
curl_setopt($ch, CURLOPT_COOKIEJAR, 'cookie.txt');
curl_setopt($ch, CURLOPT_POSTFIELDS, $post_data);
$result = curl_exec($ch);

源码出处：http://ericxu131.iteye.com/blog/363664

PHP发送POST请求的类

class Request{

    public static function post($url, $post_data = '', $timeout = 5){

        $ch = curl_init();

        curl_setopt ($ch, CURLOPT_URL, $url);

        curl_setopt ($ch, CURLOPT_POST, 1);

        if($post_data != ''){

            curl_setopt($ch, CURLOPT_POSTFIELDS, $post_data);

        }

        curl_setopt ($ch, CURLOPT_RETURNTRANSFER, 1); 

        curl_setopt ($ch, CURLOPT_CONNECTTIMEOUT, $timeout);

        curl_setopt($ch, CURLOPT_HEADER, false);

        $file_contents = curl_exec($ch);

        curl_close($ch);

        return $file_contents;

    }


    public static function post2($url, $data){

        

        $postdata = http_build_query(

            $data

        );

        

        $opts = array('http' =>

                      array(

                          'method'  => 'POST',

                          'header'  => 'Content-type: application/x-www-form-urlencoded',

                          'content' => $postdata

                      )

        );

        

        $context = stream_context_create($opts);


        $result = file_get_contents($url, false, $context);

        return $result;


    }


    public static function post3($host,$path,$query,$others=''){


        $post="POST $path HTTP/1.1\r\nHost: $host\r\n";

        $post.="Content-type: application/x-www-form-";

        $post.="urlencoded\r\n${others}";

        $post.="User-Agent: Mozilla 4.0\r\nContent-length: ";

        $post.=strlen($query)."\r\nConnection: close\r\n\r\n$query";

        $h=fsockopen($host,80);

        fwrite($h,$post);

        for($a=0,$r='';!$a;){

                $b=fread($h,8192);

                $r.=$b;

                $a=(($b=='')?1:0);

            }

        fclose($h);

        return $r;

    }
}

源码出处：http://www.simapple.com/278.html