socket_select()，请大神讲授

socket_select()， 请大神讲解
请大神帮忙解释一下 socket_select() 这个函数的作用，看php手册大致意思是阻塞的意思，请问什么情况下阻塞，什么时候继续往下执行啊，参数如下

int socket_select ( array &$read , array &$write , array &$except , int $tv_sec [, int $tv_usec = 0 ] );

//socket_select() accepts arrays of sockets and waits for them to change status. Those coming with BSD sockets background //will recognize that those socket resource arrays are in fact the so-called file descriptor sets. Three independent arrays of //socket resources are watched.

//param

/*
read
The sockets listed in the read array will be watched to see if characters become available for reading (more precisely, to see if a read will not block - in particular, a socket resource is also ready on end-of-file, in which case a socket_read() will return a zero length string).

write
The sockets listed in the write array will be watched to see if a write will not block.

except
The sockets listed in the except array will be watched for exceptions.

tv_sec
The tv_sec and tv_usec together form the timeout parameter. The timeout is an upper bound on the amount of time elapsed before socket_select() return. tv_sec may be zero , causing socket_select() to return immediately. This is useful for polling. If tv_sec is NULL (no timeout), socket_select() can block indefinitely.
*/

------解决方案--------------------
你从哪里看出是阻塞的？
socket_select 接受三个套接字数组，分别检查数组中的套接字是否处于可以操作的状态（返回时只保留可操作的套接字）
使用最多的是 $read，因此以读为例
在套接字数组 $read 中最初应保有一个服务端监听套接字
每当该套接字可读时，就表示有一个用户发起了连接。此时你需要对该连接创建一个套接字，并加入到 $read 数组中
当然，并不只是服务端监听的套接字会变成可读的，用户套接字也会变成可读的，此时你就可以读取用户发来的数据了
socket_select 只在套接字数组发生了变化时才返回。也就是说，一旦执行到 socket_select 的下一条语句，则必有一个套接字是需要你操作的