Context: When you use wp_insert_attachment with a relative file path, WordPress internally converts this relative path into an absolute path and stores it in the _wp_attached_file post meta field. By storing the absolute path in the _wp_attached_file meta field, WordPress ensures that it can accurately locate and access the attachment file when needed.

So, the main difference is in the SQL query:

attachment_url_to_postid():

$sql = $wpdb->prepare( "SELECT post_id, meta_value FROM $wpdb->postmeta WHERE meta_key = '_wp_attached_file' AND meta_value = %s", $path );

custom function:

$sql = $wpdb->prepare( "SELECT ID FROM {$wpdb->posts} WHERE guid = %s", $url );

Then the _wp_attached_file has the full path and relative path for this we need to check the posts table where we have the guid.

So, to conclude the attachment_url_to_postid() has better efficiency & ease of use, but it doesn’t cover 100% of the cases so it’s reliability is less that the custom function.

I also think that `guid` does not have to be a URL at all. I’m not sure about this.