PHP replace & with &amp; not &amp;amp;
<p>At the moment I have:</p>

<pre><code>//replace common characters
$search = array('&amp;', '£', '$'); 
$replace = array('&amp;amp;', '&amp;pound;', '&amp;dollar;'); 
$html= str_replace($search, $replace, $html);
</code></pre>

<p>The problem with this code is that if the &amp; has been already converted it will try to convert it again. How would I make sure this doesn't happen?</p>
