GetCallback — Returns the current callback function registered for the given reason
$plot->GetCallback($reason)
GetCallback returns the current callback function
registered for the given reason.
That is, it returns the function argument value used when a callback
function was registered with SetCallback.
Returns the function name as a string, or a 2-element array for object instance and method calls. Returns False if there is no callback registered for this reason, or if the given reason is not valid.
Since no valid function name (or array of object instance and method) evaluates to false when directly tested, it is not necessary to check using the identical-to (===) operator.
Callbacks are an experimental feature added to PHPlot-5.0.4. Refer to Chapter 9, Callbacks for more information.