When trying to create excel file I get Warning: Cannot modify header information

Question

I have a PHP code to export the data from mysql to excel sheet.

export_customer.php

<?php
         include 'psl-config.php';
         function export_excel_csv(){

      $cust_name = $_GET['msg'];
      $conn = mysql_connect(HOST, USER, PASSWORD);
      $db = mysql_select_db(DATABASE,$conn); 
      $sql = "Select t1.custi_id_fk, t2.full_name, t2.phone_1, t2.phone_2, t1.amount from outwards_credit As t1 INNER JOIN cust_info As t2 ON t1.custi_id_fk = t2.cust_id where t1.bill_state = 0 and t1.custi_id_fk = ". $cust_name;

     $rec = mysql_query($sql) or die (mysql_error());

      $num_fields = mysql_num_fields($rec);

      for($i = 0; $i < $num_fields; $i++ )
      {
          $header .= mysql_field_name($rec,$i)."\\t";
      }

      while($row = mysql_fetch_row($rec))
      {
          $line = '';
          foreach($row as $value)
          {                                            
              if((!isset($value)) || ($value == ""))
              {
                  $value = "\\t";
              }
              else
              {
                  $value = str_replace( '"' , '""' , $value );
                  $value = '"' . $value . '"' . "\\t";
              }
              $line .= $value;
          }
          $data .= trim( $line ) . "\\n";
      }

      $data = str_replace("\\r" , "" , $data);

      if ($data == "")
      {
          $data = "\\n No Record Found!\n";                        
      }
      header("Content-type: application/octet-stream");
      header("Content-Disposition: attachment; filename=reports.xls");
      header("Pragma: no-cache");
      header("Expires: 0");
      print "$header\\n$data";
  }
       if(isset($_GET['msg'])){ 

    export_excel_csv();
     } else {
         echo "There are no data";
     }

     ?>

I call this using a href from another file:

<a href= export_customer.php?msg=1> Export </a>

But I end up getting the following errors.

Warning: Cannot modify header information - headers already sent by (output started at /includes/export_customer.php:1) in /includes/export_customer.php on line 45

Warning: Cannot modify header information - headers already sent by (output started at /includes/export_customer.php:1) in /includes/export_customer.php on line 46

Warning: Cannot modify header information - headers already sent by (output started at /includes/export_customer.php:1) in /includes/export_customer.php on line 47

Warning: Cannot modify header information - headers already sent by (output started at /includes/export_customer.php:1) in /includes/export_customer.php on line 48 custi_id_fk\tfull_name\tphone_1\tphone_2\tamount\t\n"1"\t"customer"\t"123123"\t"987987"\t"100.00"\t\n"1"\t"customer"\t"123123"\t"987987"\t"10000.00"\t\n

Where am I going wrong???