imagesettile() sets the tile image to be used by all region filling functions (such as imagefill() and imagefilledpolygon()) when filling with the special color IMG_COLOR_TILED.
A tile is an image used to fill an area with a repeated pattern. Any GD image can be used as a tile, and by setting the transparent color index of the tile image with imagecolortransparent(), a tile allows certain parts of the underlying area to shine through can be created.
Note: You need not take special action when you are finished with a tile, but if you destroy the tile image, you must not use the IMG_COLOR_TILED color until you have set a new tile image!