title: base_convert and negative numbers
body: <p>The base_convert() function doesn't seem to preserve the sign.  </p>
  
  <p>For example: </p>
  
  <pre><code>var_dump (base_convert ('-100', 10, 10));
  </code></pre>
  
  <p>The output of this is 100</p>
  
  <p>Is there a way of converting bases without losing the sign?</p>