php CURL中这个参数如何理解？

Question

我想问下CURLOPT_HEADERFUNCTION和CURLOPT_WRITEFUNCTION参数如何理解？我不知道怎么使用，php手册上说的很模糊不懂。

这是github上找到的一个例子，但是使用的时候并没有给$string传值啊，这个值是从何而来？

<?php 
 $ch = curl_init();  
curl_setopt($ch, CURLOPT_URL, 'http://www.php.net/'); 
// Set callback function for headers 
curl_setopt($ch, CURLOPT_HEADERFUNCTION, 'read_header'); 
// Set callback function for body 
curl_setopt($ch, CURLOPT_WRITEFUNCTION, 'read_body');  
curl_exec($ch);  
if ($error = curl_error($ch)) {     
  echo "Error: $error<br />\n";
}  
 function read_header($ch, $string) { 
    $length = strlen($string);     
    echo "Header1: $string<br />\n";     
    return $length; 
  } 
  
 function read_body($ch, $string) {     
    $length = strlen($string);     
    echo "Received $length bytes<br />\n";     
    return $length; 
  }  
  ?>

Answer 1

CURLOPT_HEADERFUNCTION A callback accepting two parameters. The first is the cURL resource, the second is a string with the header data to be written. The header data must be written by this callback. Return the number of bytes written.
http://cn2.php.net/manual/en/...