先看看fread的manual，如下：

http://php.net/manual/en/function.fread.php

fread() reads up to length bytes from the file pointer referenced by handle. Reading stops as soon as one of the following conditions is met:

  • length bytes have been read
  • EOF (end of file) is reached
  • a packet becomes available or the socket timeout occurs (for network streams)
  • if the stream is read buffered and it does not represent a plain file, at most one read of up to a number of bytes equal to the chunk size (usually 8192) is made; depending on the previously buffered data, the size of the returned data may be larger than the chunk size.

中文：

fread() 从文件指针 file 读取最多 length 个字节。该函数在读取完最多 length 个字节数，或到达 EOF 的时候，或（对于网络流）当一个包可用时，或（在打开用户空间流之后）已读取了 8192 个字节时就会停止读取文件，视乎先碰到哪种情况。

返回所读取的字符串，如果出错返回 false。

 

结论：大家要注意上面红色的地方，一定要判断fread的返回值。我就是没有看文档，以为需要多少，就能读到多少。结果当读取的字节数过大时（与chunk size有关，好像是4K），各种出错。（这也与python的误导有关，因为python的sys.stdin.read就不是这样，我是参考了python的写法）

参考如下代码：

$v_content = '';
    while (strlen($v_content) < $v_len[1]) {
        $v_content .= fread(STDIN, $v_len[1] - strlen($v_content));
    }