Mine

function detect_ie6()
{
    if (isset($_SERVER['HTTP_USER_AGENT']) && 
    (strpos($_SERVER['HTTP_USER_AGENT'], 'MSIE 6') !== false))
        return true;
    else
        return false;
}

if (detect_ie6()) { 
	echo "logo.gif";
}
else {
	echo "logo.png";
}