Insert thumbnail (attachment) post by url

readme.md

Example code

$url = "http://i.imgur.com/LNWnPxA.png";
$post_ID = "1"; // Post number ID

$image_id = wp_insert_attachment_by_url( $url, $post_ID );

if ( ! is_wp_error( $image_id ) ) {
  /**
   * Show ID Thumbnail
   * Use "wp_get_attachment_url" to get url.
   */
	print_r( $image_id ); // Show ID Thumbnail
} else {
	print_r( $image_id ); // Show Error
}

functions.php

<?php
/**
 * Insert thumbnail by URL
 * Is very good to execute wp_insert_post
 * 
 * @author Lenivene Bezerra
 * @param  string $url     Image url external or no
 * @param  int    $post_ID Post id to insert thumbnail
 * @return ID Thumbnail | WP_Error
 */
function wp_insert_attachment_by_url( $url, $post_ID ) {
	require_once( ABSPATH . 'wp-admin/includes/media.php' );
	require_once( ABSPATH . 'wp-admin/includes/file.php' );
	require_once( ABSPATH . 'wp-admin/includes/image.php' );

	if ( '' == $post_ID )
		return new WP_Error( 'insert_attachment_failed', __( 'Invalid post ID' ) );

	if( !empty( $url ) ){
		$url = esc_url( $url );

		/**
		 * Set variables for storage, fix file
		 * filename for query strings.
		 */
		preg_match( '/[^\?]+\.(jpe?g|jpe|gif|png)\b/i', $url, $matches );
		if ( ! $matches ) {
			return new WP_Error( 'insert_attachment_failed', __( 'Invalid image URL' ) );
		}

		$file_array = array();
		$file_array['name'] = basename( $matches[0] );

		/**
		* Download file to temp location.
		*/
		$file_array['tmp_name'] = download_url( $url );

		/**
		* Check for download errors
		* if there are error unlink the temp file name
		*/
		if ( is_wp_error( $file_array['tmp_name'] ) ) {
			return $file_array['tmp_name'];
		}

		$image_ID = media_handle_sideload( $file_array, $post_ID );

		/**
		 * If error storing permanently, unlink.
		 */
		if ( is_wp_error( $image_ID ) ) {
			@unlink( $file_array['tmp_name'] );
			return $image_ID;
		}

		/**
		 * If error thumbnail
		 */
		if ( false === set_post_thumbnail( $post_ID, $image_ID ) ) {
			return new WP_Error( 'insert_attachment_failed', __( 'Problem to set post thumbnail' ) );
		}

		return $image_ID;
	}else{
		return new WP_Error( 'insert_attachment_failed', __( 'Insert URL' ) );
	}
}