Upload image by url

functions.php

<?php

function wp_upload_image_by_url( $url, $post_ID = 0 ) {
	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( 'upload_image_by_url_failed', __( 'Invalid post ID' ) );

	if( empty( $url ) ){
		return new WP_Error( 'upload_image_by_url_failed', __( 'Insert URL' ) );
	}
	else{
		/**
		 * Sanitizing URLs
		 */
		$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( 'upload_image_by_url_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;
		}

		return $image_ID;
	}
}