How to store image url in database?

Question

WordPress

I use a regular ajax form to send files as pictures

add_action( 'wp_ajax_comment_upload_image', 'upload_image' );
add_action( 'wp_ajax_nopriv_comment_upload_image', 'upload_image' );

function upload_image() {
  if ( !isset( $_FILES['files'] ) && empty( $_FILES['files'] ) ) {
    return;
  }

  $img = file_get_contents( $_FILES['files']['tmp_name'] );
}

How can the url of the resulting image be stored in the wp_comment_image table?
I will have 2 fields: comment_id (by default null) and url (by default, which is better to set?)