generate a post_thumbnail (and upload) from a url.

wp-thumbnail-from-url.php

<?php
  function upload_thumbnail($image_url, $postID) {
        // Add Featured Image to Post
        $upload_dir = wp_upload_dir(); // Set upload folder
        $image_data = file_get_contents($image_url); // Get image data
        $filename   = basename($image_url); // Create image file name
        // $filename   = strstr($filename, '?', true); // we stripped the cache from instagram


        // Check folder permission and define file location
        if( wp_mkdir_p( $upload_dir['path'] ) ) {
            $file = $upload_dir['path'] . '/' . basename($filename);
        } else {
            $file = $upload_dir['basedir'] . '/' . basename($filename);
        }

        // Create the image  file on the server
        file_put_contents( $file, $image_data );

        // Check image file type
        $wp_filetype = wp_check_filetype( $filename, null );

        // Set attachment data
        $attachment = array(
            'post_mime_type' => $wp_filetype['type'],
            'post_title'     => sanitize_file_name( $filename ),
            'post_content'   => '',
            'post_status'    => 'inherit'
        );

        // Create the attachment
        $attach_id = wp_insert_attachment( $attachment, $file, $postID );

        require_once(ABSPATH . 'wp-admin/includes/image.php');
        $attach_data = wp_generate_attachment_metadata( $attach_id, $file );

        wp_update_attachment_metadata( $attach_id, $attach_data );
        set_post_thumbnail( $postID, $attach_id );
    }