title: Change the href attribute of <a> tags in an html document
body: <p>I'm using <code>file_get_contents</code> to get a certain file's contents -- so far that is working.</p>
  <p>Now I want to search the file and replace all <code>&lt;a href=&quot;</code> with  <code>&lt;a href=&quot;site.php?url=</code> before showing the file.</p>
  <p>How can I do this? I know I should use some kind of <code>str_replace</code> or even <code>preg_replace</code>. But I don't know how to actually search and do it for the file I'm getting with <code>file_get_contents</code>.</p>