force download as csv

I am trying to force results from a mysql table as a csv and I am having issues as the format is all messed up. if i do the same thing to a .xls file it comes out perfectly. Please tell me what I am doing wrong. Thanks

<?php
ob_start();
session_start();
include ("includes/db.php");
include ("includes/functions.php");
$storeconn = dbconn(2);
$sql = stripslashes($_POST['sql']);
$file = $_POST['file'];        
$export = mysql_query($sql);
$fields = mysql_num_fields($export);

for ($j = 0; $j < $fields; $j++) {
    $header .= mysql_field_name($export, $j) . "\t";
}
$cnt = 0;
while($row = mysql_fetch_row($export)) {
    if ($cnt > 0) {
        $line = '';
        foreach($row as $value) {                                            
            if ((!isset($value)) OR ($value == "")) {
                $value = "\t";
            } else {
                $value = str_replace('"', '""', $value);
                $value = '"' . $value . '"' . "\t";
            }
            $line .= $value;
        }
        $data .= trim($line)."\n";
    }
    $cnt++;
}
$filename = $file.".xls";
header("Content-type: application/octet-stream");
header("Content-Disposition: attachment; filename=".$filename);
header("Pragma: no-cache");
header("Expires: 0");
print "$header\n$data";
mysql_close($storeconn);
ob_flush();
?>