PHP method to realize three-level linkage of year, month and day

<input type="text" id="hs"> <input type="button" id="btn" value="提交">
<span id="rent"></span>

var str = "<select id=&#39;year&#39;></select>  <select id=&#39;month&#39;>" +
    "</select>  <select id=&#39;day&#39;></select>";
  $("#rent").html(str);

  fullyear();
  fullmonth();
  fullday();

  //当其中一个改变，后面的要跟着改变

  $("#year").change(function(){

    fullday();
  });
  $("#month").change(function(){

    fullday();
  });


  function fullyear()

  {
    var d = new Date();
    var year = d.getFullYear();
    str ="";
    for(var i=year-5;i<year+6;i++)
    {
      if(i==year)
      {
        str += "<option selected=&#39;selected&#39; value=&#39;"+i+"&#39;>"+i+"</option>";
      }
      else {
        str +="<option value=&#39;"+i+"&#39;>"+i+"</option>"
      }
    }
    $("#year").html(str);
  }

  function fullmonth()
  {
    var d = new Date();
    var month = d.getMonth()+1;
    str ="";
    for(var j=1;j<13;j++)
    {
      if(j==month)
      {
        str += "<option selected=&#39;selected&#39; value=&#39;"+j+"&#39;>"+j+"</option>";
      }
      else {
        str +="<option value=&#39;"+j+"&#39;>"+j+"</option>"
      }
    }
    $("#month").html(str);

  }

  function fullday()
  {
    var d = new Date();
    var day = d.getDate();
    var year=$("#year").val();
    var month=$("#month").val();
    var rq=31;
    str ="";
    if(month==4|| month==6|| month==9|| month===11)
    {
      rq=30;
    }
    else if(month==2)
    {
      if(year%4==0 && year%100!=0 || year%400==0)
      {
        rq=29;//闰年
      }
      else{
        rq=28; //不是闰年
      }
    }
    for(var n=1;n<rq+1;n++)
    {
      if(n==day)
      {
        str +="<option selected=&#39;selected&#39; value=&#39;"+n+"&#39;>"+n+"</option>";
      }
      else
      {
        str +="<option value=&#39;"+n+"&#39;>"+n+"</option>";
      }
    }

    $("#day").html(str);

  } //到这里就完成了下拉列表的内容了，下一步要做的是把内容存到表单中

$("#btn").click(function(){
 var nian=$("#year").val();
 var yue=$("#month").val();
 var ri=$("#day").val();

var time=nian+"-"+yue+"-"+ri+"";

 $("#hs").val(time)


})