How to save encoded Base64 strings from database, to files, on disk?

First post:

I have a MySQL database, which is created by this command:

CREATE TABLE IF NOT EXISTSCKox(aTEXT NOT NULL,bTEXT NOT NULL );

Below is its structure:

+----------+-----------------+
|    id    |   base64_str    |
+----------+-----------------+
|  file_1  |  base64_str_1   |
|  file_2  |  base64_str_2   |
|  file_3  |  base64_str_3   |
|  file_4  |  base64_str_4   |
+----------+-----------------+

To save these Base64 strings (in database) to files (on my web server), I have run this PHP code:

<?php

    // database connection
    // ...

    $cmd = mysql_query("SELECT * FROM `MyTable`");
    while($data = mysql_fetch_array($cmd)) {

    $filename = $data[id];
    $filedata = $data[base64_str];

    header('Content-Description: File Transfer');
    header("Content-type: application/octet-stream");
    header("Content-disposition: attachment; filename= ".$filename."");

    echo base64_decode($filedata);

    }

    // ...

?>

It return only one file, and there is no data in this saved file. But, I want to save these Base64 strings to files, on server disk.

After getting some helps from people (on StackOverlow), I have updated my code, it is full:

// database connection
// ...

$cmd = mysql_query("SELECT * FROM `MyTable`");

while ($data = mysql_fetch_array($cmd)) {

file_put_contents($data['id'], base64_decode($data['base64_str']));
echo $data['base64_str'];

}

// ...

But, I still get one file only. What is the main problem? How to solve it?