media_sideload_image( string $file, int $post_id, string $desc = null, string $return = 'html' )

function media_sideload_image( $file, $post_id, $desc = null, $return = 'html' ) {
	if ( ! empty( $file ) ) {

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

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

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

		// If error storing temporarily, return the error.
		if ( is_wp_error( $file_array['tmp_name'] ) ) {
			return $file_array['tmp_name'];
		}

		// Do the validation and storage stuff.
		$id = media_handle_sideload( $file_array, $post_id, $desc );

		// If error storing permanently, unlink.
		if ( is_wp_error( $id ) ) {
			@unlink( $file_array['tmp_name'] );
			return $id;
		// If attachment id was requested, return it early.
		} elseif ( $return === 'id' ) {
			return $id;
		}

		$src = wp_get_attachment_url( $id );
	}

	// Finally, check to make sure the file has been saved, then return the HTML.
	if ( ! empty( $src ) ) {
		if ( $return === 'src' ) {
			return $src;
		}

		$alt = isset( $desc ) ? esc_attr( $desc ) : '';
		$html = "<img src='$src' alt='$alt' />";
		return $html;
	} else {
		return new WP_Error( 'image_sideload_failed' );
	}
}