PHP：将数组放入文件时在$ _POST元素之间添加逗号

The following is my code:

<?php
    #put the info the user just inputed into a file for later retrival
    file_put_contents("test.txt",$_POST, FILE_APPEND);
?>

And this is the result I get in test.txt:

JaneF16xyzWindows1215

How can I change the format of the result to be

Jane,F,16,xyz,Windows,12,15

(have commas between the elements)

Change it to

file_put_contents('test.txt', implode(',', $_POST), FILE_APPEND);

Note that this does not consider the possibility of array post values.