This allows you to take something from the $_FILES array on user input and insert it into a post. as featured or non featured image.

insert_attachment.php

<?php
function insert_attachment($file_handler,$post_id,$setthumb='false') {
    // check to make sure its a successful upload
    if ($_FILES[$file_handler]['error'] !== UPLOAD_ERR_OK) __return_false();
     
    require_once(ABSPATH . "wp-admin" . '/includes/image.php');
    require_once(ABSPATH . "wp-admin" . '/includes/file.php');
    require_once(ABSPATH . "wp-admin" . '/includes/media.php');
     
    $attach_id = media_handle_upload( $file_handler, $post_id );
     
    if ($setthumb) update_post_meta($post_id,'_thumbnail_id',$attach_id);
    return $attach_id;
}