PHP+MYSQL导出所有行


<?php
$mysqli = new mysqli('localhost', 'root', 'bmmepmhepb11619602VASQE', 'corporate');

$query = 'SELECT * from sales WHERE sub_total=21.00';
$result = mysqli_query($mysqli, $query);
if (mysqli_num_rows($result) != false) {
$fp = fopen('D:\xampp\mysql\data\corporate\1207.txt', 'a+');
if ($fp != false) {
$row_num = mysqli_num_rows($result);

for ($row = 0; $row < $row_num; $row++) {
while (($r = mysqli_fetch_assoc($result)) != false)
file_put_contents('D:\xampp\mysql\data\corporate\1207.txt', join(', ', $r));
}
}
fclose($fp);
}

mysqli_close($mysqli);
?>
mysql:
+----+-----------+---------------------+-----------+---------------+------------
+
| id | client_id | order_time          | sub_total | shipping_cost | total_cost
|
+----+-----------+---------------------+-----------+---------------+------------
+
|  1 |      2442 | 2012-10-10 23:23:35 |    651.00 |       5631.00 |  651651.00
|
|  3 |      2446 | 2012-10-27 00:33:46 |     21.00 |         54.00 |      75.00
|
|  4 |      2446 | 2012-10-27 00:34:13 |     21.00 |         54.00 |      75.00
|
+----+-----------+---------------------+-----------+---------------+------------为什么只导出最后一行？如何导出所有行？