preg_replace有关问题，求帮助

$wupin="A类别0::::::品名1::::::品牌/厂商2::::::规格3::::::要改变的数量4::::::单位5::::::单价6::::::要改变的总价7|||||||B类别0::::::品名1::::::品牌/厂商2::::::规格3::::::要改变的数量4::::::单位5::::::单价6::::::要改变的总价7";

$strrpos_start="A类别0::::::品名1::::::品牌/厂商2::::::规格3::::::";//查询前面部分
$strrpos_end="单位5::::::单价6::::::";//查询后面部分

$k =$strrpos_start;
$n =10;
echo preg_replace("/(?<=$k )\d+/e", '0 + '.$n, $wupin);//行不通，出错
?>

$wupin="A类别0::::::品名1::::::品牌/厂商2::::::规格3::::::要改变的数量4::::::单位5::::::单价6::::::要改变的总价7
------解决思路----------------------
B类别0::::::品名1::::::品牌/厂商2::::::规格3::::::要改变的数量4::::::单位5::::::单价6::::::要改变的总价7";
 
$ep = explode('
------解决思路----------------------
', $wupin);
if($ep[0][0] == 'A'){
  $ep[0] = preg_replace_callback("/(要改变的数量)(\d)/", create_function('$matches','return $matches[1].($matches[2]+10);'), $ep[0]);
}
echo join($ep);