I'm trying to fix an old plugin that is giving me a php error. I've work with website developer for years but my knowledge of php is limited. If anyone can get me advice I would appreciate it.

The orginal plugin is Upload images to remote server

Usage: Lets you use an FTP-Server as media-server in WordPress.

When I check the php code at I get error message

Error(s), warning(s):
PHP Fatal error:
Uncaught Error: Call to undefined function add_filter() in 2062826375/source.php:34
Stack trace: #0 {main} thrown in 2062826375/source.php on line 34

Line 34 is add_filter( 'wp_handle_upload', 'get_file_name' );

Any thoughts on how to fix this? Thank you Kimberly