ThinkPHP 三级联动

<div id="cBox">
<select id="pro">
<option>--请选择省份--</option>
</select>
</div>

$(function(){
$.ajax({type:'GET', url:'__URL__/linkage', data:'upid=0',
success:function( data ){
var proOption = '';//用来存放省份的选项
for( var i=0;i<data.length;i++){
proOption += '<option value='+data[i].id+'>'+data[i].name+'</option>';
}
$('#pro').append(proOption);
}, dataType:'json'
});
$('#cBox').on('change','select',function(){
var ob = $(this);
ob.nextAll('select').remove();
var val = $(this).val();
$.get('__URL__/linkage', {upid:val}, function(res){
if( res.length <=0 ) return false;
var len = $('#cBox select').length;

switch( len ){
case 1:
idValue = 'city';
break;
case 2:
idValue = 'qu';
break;
case 3:
idValue = 'xian';
break;
case 4:
idValue ='dao';
break;
default:
idValue = '222';
break;
}
var select = '<select id="'+idValue+'"><option>--请选择--</option>';
for( var i=0;i<res.length;i++){
select += '<option value="'+res[i].id+'">'+res[i].name+'</option>';
}
select +='</select>';
ob.after( select );
},'json'
);
});
});

public function linkage() {
$map['pid'] = I('get.upid');
$cityList = M('category')->where($map)->select();
echo json_encode($cityList);
}