VIPs Download Remote Image and create attachment in WordPress.

gistfile1.php

<?php

/**
 * Downloads an external image and optionally attaches it to a post.
 *
 * Contains most of core's media_sideload_image() but returns an attachment ID instead of HTML.
 *
 * Note: this function does not validate the domain that the image is coming from. Please make sure
 *   to validate this before downloading the image. Should only pull down images from trusted sources.
 *
 * @param string $image_url URL of the image.
 * @param int $post_ID ID of the post it should be attached to.
 * @return $thumbnail_id id of the thumbnail attachment post id
 */
function wpcom_vip_download_image( $image_url, $post_id = 0, $description = '' ) {
	if ( isset( $_SERVER['REQUEST_METHOD'] ) && strtoupper( $_SERVER['REQUEST_METHOD'] ) == 'GET' ) {
		return new WP_Error( 'invalid-request-method', 'Media sideloading is not supported via GET. Use POST.' );
	}

	if ( ! is_admin() ) {
		return new WP_Error( 'not-in-admin', 'Media sideloading can only be done in when `true === is_admin()`.' );
	}

	if ( $post_id < 0 ) {
		return new WP_Error( 'invalid-post-id', 'Please specify a valid post ID.' );
	}

	if ( ! filter_var( $image_url, FILTER_VALIDATE_URL ) ) {
		return new WP_Error( 'not-a-url', 'Please specify a valid URL.' );
	}

	$image_url_path = parse_url( $image_url, PHP_URL_PATH );
	$image_path_info = pathinfo( $image_url_path );

	if ( ! in_array( strtolower( $image_path_info['extension'] ), array( 'jpg', 'jpe', 'jpeg', 'gif', 'png' ) ) ) {
		return new WP_Error( 'not-an-image', 'Specified URL does not have a valid image extension.' );
	}

	// Download file to temp location; short timeout, because we don't have all day.
	$downloaded_url = download_url( $image_url, 30 );

	// We couldn't download and store to a temporary location, so bail.
	if ( is_wp_error( $downloaded_url ) ) {
		return $downloaded_url;
	}

	$file_array['name'] = $image_path_info['basename'];
	$file_array['tmp_name'] = $downloaded_url;

	if ( empty( $description ) ) {
		$description = $image_path_info['filename'];
	}

	// Now, let's sideload it.
	$attachment_id = media_handle_sideload( $file_array, $post_id, $description );

	// If error storing permanently, unlink and return the error
	if ( is_wp_error( $attachment_id ) ) {
		@unlink( $file_array['tmp_name'] ); // unlink can throw errors if the file isn't there
		return $attachment_id;
	}

	return $attachment_id;
}