Fastest way to implode an associative array with keys

I'm looking for a fast way to turn an associative array in to a string. Typical structure would be like a URL query string but with customizable separators so I can use '&' for xhtml links or '&' otherwise.

My first inclination is to use foreach but since my method could be called many times in one request I fear it might be too slow.

<?php
$Amp = $IsXhtml ? '&amp;' : '&';
$Parameters = array('Action' => 'ShowList', 'Page' => '2');
$QueryString = '';
foreach ($Parameters as $Key => $Value)
        $QueryString .= $Amp . $Key . '=' . $Value;

Is there a faster way?