Since OOH Forms is object oriented, it can be easily customized by extending the classes that define the element types. In general, you must make sure your derived class has a constructor and you may override any of the self_* functions of of_element. The source for the existing elements is the best documentation for how to do this properly, but a few general notes follow.
Display an instance of this element unfrozen. $val is the $value argument of show_element if there was one; $which can be used as an index for array valued elements; it is equal to the number of times show_element has been called for this element previously. This function must return the number of hidden tags output.
Display an instance of this element frozen. In addition to the html to show the frozen element, you must output tags for hidden fields to duplicate the effect of submitting an unfrozen element of this type. The function must return the number of hidden tags output;
Validate $val for this element. If it is valid, return false, otherwise return the relevant error string.
Print out Javascript code to validate this element. $ndx_array is an array of the indices of the elements to be validated as used in the Javascript form.element[] array. This is needed since the extra [] in array valued element names confuses Javascript.