php的exec执行后台进程会堵塞，卡到子进程结束才结束

贵贵的博客 ( http://blog.linuxphp.org/ ) : php入口执行一个shell

<?php 
$a = exec("./a.sh");
var_dump($a);

  看下a.sh 的内容

#!/bin/bash
echo "11"
(sleep 10) &

shell的sleep是后台执行的，php执行shell，php会卡住等到10s才结束，为什么？   --- 经过朋友的提醒发现要把sleep的输出重定向到 /dev/null 修改后脚本如下

#!/bin/bash
echo "11"
(sleep 10 > /dev/null) &

  官方也有给相关的说明，只是在很不起眼的Note 

Note:

If a program is started with this function, in order for it to continue running in the background, the output of the program must be redirected to a file or another output stream. Failing to do so will cause PHP to hang until the execution of the program ends.