Clean Media Library from broken images. Delete attachments which files no longer available and return 404 error [Wordpress]

delete_404_attachments.php

function delete_404_attachments(){
	$attachments = get_posts( array(
	'post_type' => 'attachment',						
	'numberposts' => -1,	 	  								
	'fields' => 'ids'
		));									
		if ($attachments) {		 		
			foreach ($attachments as $attachmentID){
				$file_url = wp_get_attachment_url( $attachmentID); 												
				$file_headers = @get_headers($file_url);
				if($file_headers[0] == 'HTTP/1.1 404 Not Found') {
					$deleted = wp_delete_attachment($attachmentID, true);
				}				
			}
		}					
}

Not used anything like this before - where would I add this? to the funcitons.php file? then how would I run it?

good question

You can action init.