聊好的fsockopen的非阻塞模式呢？为什么还是会阻塞

function fsockopen_test(){
    $hostname = 'm.cn';
    $url = '/test/1.php';
    $fp = fsockopen($hostname, 80, $errno, $errstr, 5);
    if (!$fp) {
        echo "$errstr ($errno)";
        return false;
    }
    stream_set_blocking($fp,0);//开启了手册上说的非阻塞模式
    $header = "GET $url HTTP/1.1\r\n"; 
    $header.="Host: $hostname\r\n";
    $header.="Connection: Close\r\n\r\n";//长连接关闭
    fwrite($fp, $header);
    fclose($fp);
}
fsockopen_test();

sleep(6);
$fp  =  fopen ( './data.txt' ,  'w' );
fwrite ( $fp ,111);
fclose ( $fp );

echo date('Y-m-d H:i:s'), PHP_EOL;
fsockopen_test();
echo date('Y-m-d H:i:s'), PHP_EOL;