PHP如何实现简单爬虫的方法

<?php
/**
 * 爬虫程序 -- 原型
 *
 * 从给定的url获取html内容
 * 
 * @param string $url 
 * @return string 
 */
function _getUrlContent($url) {
  $handle = fopen($url, "r");
  if ($handle) {
    $content = stream_get_contents($handle, 1024 * 1024);
    return $content;
  } else {
    return false;
  } 
} 
/**
 * 从html内容中筛选链接
 * 
 * @param string $web_content 
 * @return array 
 */
function _filterUrl($web_content) {
  $reg_tag_a = '/<[a|A].*?href=[\'\"]{0,1}([^>\'\"\ ]*).*?>/';
  $result = preg_match_all($reg_tag_a, $web_content, $match_result);
  if ($result) {
    return $match_result[1];
  } 
} 
/**
 * 修正相对路径
 * 
 * @param string $base_url 
 * @param array $url_list 
 * @return array 
 */
function _reviseUrl($base_url, $url_list) {
  $url_info = parse_url($base_url);
  $base_url = $url_info["scheme"] . '://';
  if ($url_info["user"] && $url_info["pass"]) {
    $base_url .= $url_info["user"] . ":" . $url_info["pass"] . "@";
  } 
  $base_url .= $url_info["host"];
  if ($url_info["port"]) {
    $base_url .= ":" . $url_info["port"];
  } 
  $base_url .= $url_info["path"];
  print_r($base_url);
  if (is_array($url_list)) {
    foreach ($url_list as $url_item) {
      if (preg_match('/^http/', $url_item)) {
        // 已经是完整的url
        $result[] = $url_item;
      } else {
        // 不完整的url
        $real_url = $base_url . '/' . $url_item;
        $result[] = $real_url;
      } 
    } 
    return $result;
  } else {
    return;
  } 
} 
/**
 * 爬虫
 * 
 * @param string $url 
 * @return array 
 */
function crawler($url) {
  $content = _getUrlContent($url);
  if ($content) {
    $url_list = _reviseUrl($url, _filterUrl($content));
    if ($url_list) {
      return $url_list;
    } else {
      return ;
    } 
  } else {
    return ;
  } 
} 
/**
 * 测试用主程序
 */
function main() {
  $current_url = "http://hao123.com/"; //初始url
  $fp_puts = fopen("url.txt", "ab"); //记录url列表
  $fp_gets = fopen("url.txt", "r"); //保存url列表
  do {
    $result_url_arr = crawler($current_url);
    if ($result_url_arr) {
      foreach ($result_url_arr as $url) {
        fputs($fp_puts, $url . "\r\n");
      } 
    } 
  } while ($current_url = fgets($fp_gets, 1024)); //不断获得url
} 
main();
?>