CodeIgniter/jQuery - Ajax call returns full html page instead of my echo

I solved this by using a leading slash as suggested in the comments to my question.

$.ajax({
  type: "POST",
  url: "/planner/get_cal",
  dataType: "text",
  data: {current_month: current_month},
  success: function(msg){
    alert(msg);
  } 
});         

You can also get it by adding exit after echo in your php file like below:

function get_cal()
 {
    echo "dinosaurs";exit;
}

It will work. :)