I need to build an application wherein the admin must be able to define forms for data entry. The data to be entered is unknown to me. But the system will need to be able to support all the possible form fields (minus hidden fields, I suppose). So text areas, text fields, radios (with ability to specify what the options are), checkboxes, etc. Also, it needs to be able to support line item entries (similar to EE’s Matrix plugin).
Obviously, I don’t want to try to build this from the ground up. Are there any libraries I can use in CI to make life easier for me?
If none exist, what are some database design patterns I should consider for such a problem?
The best database-design for this is the Entity-Attribute-Value Model. For use with a FORM, essentially your form is the ENTITY, the Attribute will have a type (used for deciding how input will be captured, and data interpreted). Make sure that you index properly.
Remember that you'll only need to data to change the form, don't read from this model everytime you need to show a form. Store a flat file (.json is handy) of the finished form and read that when displaying the form.