php 函数的问题

<?php
    // iterate over entries in feed
    foreach ($sxml->entry as $entry) {
      // get nodes in media: namespace for media information
      $media = $entry->children('http://search.yahoo.com/mrss/');      
      // get video player URL
      $attrs = $media->group->player->attributes();
      $watch = $attrs['url'];
      
      // get video thumbnail
      $attrs = $media->group->thumbnail[0]->attributes();
      $thumbnail = $attrs['url'];



中这句干什么用
$media = $entry->children('http://search.yahoo.com/mrss/');

你要看类中的那个children方法是怎么写的啊