This option should contain a function name to be executed each time contents is modified by TinyMCE. This function is called each time a undo level is added to TinyMCE. The format of this function is: onchange(inst), where inst is the editor instance object reference.
Example of usage of the onchange_callback option:
function myCustomOnChangeHandler(inst) {
alert("Some one modified something");
alert("The HTML is now:" + inst.getBody().innerHTML);
}
tinyMCE.init({
...
onchange_callback : "myCustomOnChangeHandler"
});