为什么php fread()函数有时会在每一行前面加上它的长度?

我使用PHP从套接字执行POST请求。下面是一段代码:

// open a socket connection on port 80
$fp = fsockopen($host, 80);

// send the request headers:
fwrite($fp, "POST $path HTTP/1.1\r\n");
fwrite($fp, "Host: $host\r\n");
fwrite($fp, "Referer: $referer\r\n");
fwrite($fp, "Content-type: application/x-www-form-urlencoded\r\n");
fwrite($fp, "Content-length: ". strlen($data) ."\r\n");
fwrite($fp, "Connection: close\r\n\r\n");
fwrite($fp, $data);

$result = ''; 
while(!feof($fp)) {
    // receive the results of the request
    $result .= fread($fp, 5000);
}

但是,响应的每一行似乎都以其十六进制长度作为前缀。为什么会这样?

例如,我发布的测试cgi脚本返回其环境:

BASH=/bin/bash
BASHOPTS=cmdhist:extquote:force_fignore:hostcomplete:interactive_comments:progcomp:promptvars:sourcepath
BASH_ALIASES=()
BASH_ARGC=()
BASH_ARGV=()

但我的PHP函数的结果是:

f
BASH=/bin/bash

69
BASHOPTS=cmdhist:extquote:force_fignore:hostcomplete:interactive_comments:progcomp:promptvars:sourcepath

10
BASH_ALIASES=()