client.js

/**************************
 * Html form with hidden data called csv_hdr and csv_output.
 *********************************/
$(document).ready(function(){
	  $("#btn_challenge_score_details").click(function (){
	    
	        var csv_hdr = $("#csv_hdr").val();
	        var csv_output = $("#csv_output").val();
	      	var dataString = 'csv_hdr='+ csv_hdr + '&csv_output=' + csv_output;  
			//alert (dataString); return false;
	        $.ajax({
	        type: "POST",
	        url: "<?=base_url()?>challenge/export_csv",
	        data: dataString,
	        cache:false,
	        success: 
	          function(data){
	            $("#message").html(data); 
	          }
	        });
	      return false;
	    });
	});

server.php

/********
 * Controller method
 ***************************************************/
function export_csv()
{
	$out = '';

	if (isset($_POST['csv_hdr'])) 
	{
		$out .= $_POST['csv_hdr'];
		$out .= "\n";
	}

	if (isset($_POST['csv_output'])) 
	{
		$out .= $_POST['csv_output'];
	}

	$file = 'ExportCSV';
	$filename = $file."_".date("Y-m-d_H-i",time());

	//Generate the CSV file header
	header("Content-type: application/vnd.ms-excel");
	header("Content-disposition: csv" . date("Y-m-d") . ".csv");
	header("Content-disposition: attachment; filename=".$filename.".csv");

	print $out;

	exit;
}