Ajax post checkbox array to backend success but fail to detect if the array is null or not

I want to post three element to backend use ajax. There are input type name is "suppid" checkbox array name is "lang[]" and select type name is "guide". All three are successfully post to the backend use ajax. But I wrote a function before to detect if the post is null, and it failed to detect checkbox,no matter what I try I fail to detect checkbox array and only when write this I can check both three successfully.

    function check()

    {

    //$vali1=checknullnew($_POST); I can not use this function

    //if($vali1['vali']=='Y') if I use this no matter checkbox 
    //are clicked or not it will be successfully uploaded.


    if(isset($_POST['lang']) && isset($_POST['suppid']) &&isset($_POST['guide']))
   //I can only use this to successful detect three post are null or not
    {
        $result['vali']='Y';
                    $result['message']='successfully uploaded';
                    $result['redirect_url']='test4.html';
                    return $result;

                }
                else
                {

                    $result['vali']='N';

                    $result['message']='no';

                    return $result; 

                }



            }

            $response=check();

            echo json_encode($response);





//My html form is like this
<form role="form"  id="myform" >

                <div class="form-group">

                    <label>
                         suppid
                    </label>
                    <input type="text" class="form-control"  name="suppid" style="width: 120px;">
                </div>





                <div class="form-group">
                    <label >
                         lang
                    </label>   

                    <label class="checkbox-inline"><input type="checkbox"  name="lang[]" value="en">English</label>
                    <label class="checkbox-inline"><input type="checkbox"  name="lang[]" value="zh-CN">Chinese</label>  
                    <label class="checkbox-inline"><input type="checkbox"  name="lang[]" value="zh-TW">Cantonese</label>
                    <label class="checkbox-inline"><input type="checkbox"  name="lang[]" value="jp">Japanese</label>
                    <label class="checkbox-inline"><input type="checkbox"  name="lang[]" value="kr">Korean</label> 


                </div>
                <div class="form-group">     
                    <label>
                         guide
                    </label>                             
                          <select  class="form-control" name="guide" style="width: 120px;">
                               <option value="Y">yes</option>
                               <option value="N" selected>no</option>        
                           </select>              
                </div>







                <input name="submit" type="button" class="btn btn-default" id="button1" value="上传">



            </form>



//My javascript is 
<script>
    $(document).ready(function() {
    $('input[name=submit]').on('click',function(){

    var suppid = $('input[name=suppid]').val().trim();
    var lang = new Array;
    $("input[name='lang[]']:checked").each(function() {
    lang.push($(this).val().trim());
    });
    var guide = $('select[name=guide]').val().trim();



    //alert('ok');



    $.ajax({
                        type: "POST",
                        url: "test4.php",
                        data: {
                              suppid:suppid,
                              lang:lang,
                              guide:guide

                              },
                        dataType: "json", //send it along with your call
                        success: function( data ) 
                        {
                            if( data.vali == 'N' ) 
                            {



                            alert(data.message);
                            } 
                            else {
                            alert(data.message);
                            window.location.href = data.redirect_url;
                            }
                        }
            });

    });
});




    </script>

    //My detect null function is like this 
    function checknullnew($check)
            {
                $valid=0;


                foreach($check as $key => $value)
                {
                    if($key=='lang')
                    {
                        if(!issset($value)){$valid=1;}


                    }
                    else
                    {
                        if(trim($value)=="")
                        {
                            $valid=1;                                           
                        }

                    }




                }
                if($valid==0)
                {
                    $result['vali']='Y';
                    return $result;

                }
                else{
                    $result['vali']='N';
                    $result['message']='Please input complete messages';
                    return $result;

                }


            }

//also I try this function too
//I use isArray() to detect the checkbox array and use isset()to detect if the //array is null or not.