$(function(){
  $("#xueyuan").change(function(){
        var xueyuan = $("#xueyuan").val();
        $.ajax({
          url: "{url('ajax/xueyuan')}",
          type: "POST",
          data: {id:xueyuan},
          dataType: "json",
          error: function(){
            alert("异步查询失败,请查找原因！");
          },
          success: function(data){
                $("#zhuanye").empty();
                for(var i=0 in data){
                  $("#zhuanye").append("<option value="+data[i].l_zhuanye+">"+data[i].l_zhuanye+"</option>");
                }
              }
          });
  });

});

php的代码

	//专业动态查询
	public function xueyuan(){
		header('Content-Type:text/html; charset=utf8');
		$id = $_POST['id'];
		file_put_contents('aaaaa.txt', $id);
		$table = "zhuanyelesson";
		$result = model('ajax')->m_select($table, $id);
		echo json_encode($result);
	}