<?php
$p = &#39;/<img .*?src=[\&#39;|\"](.+?)[\&#39;|\"].*? alt="PHP code to extract the first picture on the page as a thumbnail" >/i&#39;;
preg_match_all($p,$str,$match);
print_r($match[1][0]);
?>

<img alt="PHP code to extract the first picture on the page as a thumbnail"http://usr.im/200x60?bg=3498DB&text=tool.lu" class="img" />
結(jié)果為：http://usr.im/200x60?bg=3498DB&text=tool.lu
  
<img alt="PHP code to extract the first picture on the page as a thumbnail"http://img.mukewang.com/user/55c877240001291602790279-80-80.jpg" class="img" />
結(jié)果為：http://img.mukewang.com/user/55c877240001291602790279-80-80.jpg