WP_Image_Editor_GD::flip( bool $horz, bool $vert )

	public function flip( $horz, $vert ) {
		$w = $this->size['width'];
		$h = $this->size['height'];
		$dst = wp_imagecreatetruecolor( $w, $h );

		if ( is_resource( $dst ) ) {
			$sx = $vert ? ($w - 1) : 0;
			$sy = $horz ? ($h - 1) : 0;
			$sw = $vert ? -$w : $w;
			$sh = $horz ? -$h : $h;

			if ( imagecopyresampled( $dst, $this->image, 0, 0, $sx, $sy, $w, $h, $sw, $sh ) ) {
				imagedestroy( $this->image );
				$this->image = $dst;
				return true;
			}
		}
		return new WP_Error( 'image_flip_error', __('Image flip failed.'), $this->file );
	}