mysql data exports in one column only in excel file

Whenever I click the button to export the data to an excel file, all the data is showing only in one column in excel. How can I make each columns separated? Please help. Thank you

Here is my code:

<?php

require_once("db.php");

$contents="ITEM CODE,ITEM DESC,UNIT CODE,UNIT DESC,PRODUCT CODE,DEPARTMENT,SUB DEPARTMENT,CLASS,SUB CLASS,COST,MARK UP,PRICE,ITEM TYPE\n";

$user_query = ibase_query('SELECT ITEM_CODE,ITEM_DESC,UNIT_CODE,UNIT_DESC,PRODUCT_CODE,DEPARTMENT,SUB_DEPARTMENT,CLASS,SUB_CLASS,COST,MARKUP,PRICE,ITEM_TYPE FROM ITEM_MASTER');

while($row = ibase_fetch_assoc($user_query))
{
$contents.=$row["ITEM_CODE"]."/t";
$contents.=$row["ITEM_DESC"]."/t";
$contents.=$row["UNIT_CODE"]."/t";
$contents.=$row["UNIT_DESC"]."/t";
$contents.=$row["PRODUCT_CODE"]."/t";
$contents.=$row["DEPARTMENT"]."/t";
$contents.=$row["SUB_DEPARTMENT"]."/t";
$contents.=$row["CLASS"]."/t";
$contents.=$row["SUB_CLASS"]."/t";
$contents.=$row["COST"]."/t";
$contents.=$row["MARKUP"]."/t";
$contents.=$row["PRICE"]."/t";
$contents.=$row["ITEM_TYPE"]."\n";
}

$contents = strip_tags($contents); 

header("Content-Disposition: attachment; filename=Sample Report ".date("d-m-Y").".xls");
print $contents;

?>

via Chebli Mohamed