This function is currently not documented; only the argument list is available.
This function is EXPERIMENTAL. The behaviour of this function, the name of this function, and anything else documented about this function may change without notice in a future release of PHP. Use this function at your own risk.
Sets the overall canvas size to be recorded with the drawing vector data. Usually this will be specified using the same size as the canvas image. When the vector data is saved to SVG or MVG formats, the viewbox is use to specify the size of the canvas image that a viewer will render the vector data on.