Woocommerce更新订单钩子“ save_post”不允许保存文件

Question

So I find this strange but maybe someone can help.
I need to save an incoming file from a remote server when the admin of the wordpress website updates the order in the backend.
The hook I'm using is

add_action( 'save_post', 'courier_save_wc_order_fields', 10, 2 );

Now everything works fine and I can update the post_meta as I need.
When it comes to saving an incoming PDF file things go south.
inside this perticular hook (only one I have ever found) I cannot save files.
Is this because this is when woocommerce is about to save to DB?

I've even stripped out all other code and left it as

add_action( 'save_post', 'courier_save_wc_order_fields', 10, 2 );

function courier_save_wc_order_fields( $post_id ) {
    file_put_contents('incommingdata.txt', print_r("This is random text",true));
}

Again, this is the only place this happens.. Is there a better hook that I won't have this issue?

I need one that checks a meta_box value in the Admin order page and updates the order meta accordingly while communicating to a remote server to save and retrieve a file.... Everything works except for this file saving.

Side note:

$path="my_file_received.pdf"
$decodedString = base64_decode($base64string); 
$fileHandle = fopen($path, "w"); 
if ($fileHandle) { 
    fwrite($fileHandle, $decodedString); 
    fclose($fileHandle); 
    return 1; 
}

Does not show an error yet the file does not exist either.

Answer 1

So I found the answer to my issue...

The save_post hook has a different URL than other hooks that I have used..

So when I normally used
file_put_contents('incommingdata.txt', print_r("This is random text",true));
I would always get the file saved within my plugin folder or theme folder or the base url.

The url for this hook is the wp-admin folder and not the base domain or plugin folder.

to fix this and make sure I save in the right folder the solution is:

 $this_dir_of_my_file = dirname(__FILE__);
 $this_dir_of_my_file = $this_dir_of_my_file . "/Example.txt";
 file_put_contents($this_dir_of_mine, print_r("This is random text in my file.",true));