Why do I lack permission for file editing in PHP?

Question

My URL:

https://*.*/editor.php?path=public_html&file=index.php

The editor.php file is:

<?php
 if ( isset ( $_POST [ 'plain' ] ) ) {
  print 'ok';
 } else {}
 $text = file_get_contents ( 'ftp://.../' . $_GET [ 'path' ] . '/' . $_GET [ 'file' ] );
?>
<form method="post"><textarea name="plain"><?php print $text; ?></textarea><input type="submit"></form>

If no submit occurred, so $_POST is empty, then it works. If submit happened, then it does not work, the error is:

Forbidden
You don't have permission to access this resource.
Additionally, a 403 Forbidden error was encountered while trying to use an ErrorDocument to handle the request.

What is the problem?

I try change permissions of the file as follows:

~/public_html/ftp - 0755
~/public_html/ftp/editor.php - 0644 change to 0777
~/public_html - 0755
~/public_html/index.php - 0644 change to 0777

It does not work.