php replace empty img to resolve IE6,IE7,IE8 Session disappear

/**
* 移除<img src="" />的HTML元素，避免IE6,IE7,IE8 Session消失
* global smarty filter
* @param string $tpl_output
* @param object $smarty
* @return string
*/
function replace_empty_img($tpl_output, &$smarty) {
  /*
   *
    replace = #
    input:
    $str = "<img src=\"\" width=\"1\" height=\"1\" />
    <br />
    <img src=\"\" width=\"2\" height=\"2\" />
    <img    width=\"3\"   src=\"     1\"  height=\"3\" />
    <img width=\"4\"   src=  height=\"4\" />
    ";
    output:
    <img src="/images_shop/space.gif"   width="1" height="1" />
    <br />
    <img src="/images_shop/space.gif"   width="2" height="2" />
    <img    width="3"   src="     1"  height="3" />
    <img src="/images_shop/space.gif"  width="4" height="4" />

   */
  $reg = "/(<img)(.*?)\s+src=(\'\'|\"\"|\"\s+\"|\s+){1}(.*?>)/i";

  $replace = IMAGES_URL . 'space.gif';
  $tpl_output = preg_replace($reg, "$1 src=\"{$replace}\" $2 $4", $tpl_output);
  return $tpl_output;
}