Warning: Cannot modify header information - headers already sent by...

I'm getting the above message as an error every time I perform certain administrative actions in my WP dashboard (e.g. deleting a post, editing code in Appearance > Editor, etc.)

It seems that this problem only arises when I add the following code to my functions.php:

<?php

add_filter("gform_post_submission_1", "gform_set_post_thumbnail");

function gform_set_post_thumbnail($entry){

    // get post ID of the created post

    $post_id = $entry["post_id"];

    // get the last image added to the post

    $attachments = get_posts(array('numberposts' => '1', 'post_parent' => $post_id, 'post_type' => 'attachment', 'post_mime_type' => 'image', 'order' => 'ASC'));

    if(sizeof($attachments) == 0)

        return; //no images attached to the post

    // set image as the post thumbnail

    set_post_thumbnail($post_id, $attachments[0]->ID);

}

I require this code as its function is to set the image uploaded in my Gform to be the Featured Image of the user-submitted post.

Would appreciate any help on this matter :)

EDIT: Resolved this myself. I simply moved the code that I quoted above to the top of functions.php.