解决ajax异步请求返回的是字符串问题

header(\'Content-Type:application/json;\');

contentType: \"application/json; charset=utf-8\",
dataType: \"json\",
$.ajax({
    url:\'../data/insert_category.php?article_type=\' + value,
    contentType: \"application/json; charset=utf-8\",
    dataType: \"json\",
    success:function(res){
    console.log(\'异步成功\');
    console.log(res);
    console.log(res.code);
    },
    error:function(){
    console.log(\'异步失败\');
    }
   })