ChronoForms v3 was written for Joomla! 1.5 and will only run on that Joomla! version (which is now past it's formal 'end of life').
ChronoForms v4 was wriitten for Joomla! 1.6/1.7/2.5 though there is also a version for Joomla! 1.5
On Joomla! 1.5 CF v3 requires MooTools v 1.12 while CFv4 requires the Joomla! MooTools Upgrade plug-in to be enabled so that MooTools 1.2 is loaded.
The basic concepts of ChronoForms are the same in both versions but the Admin interface in CFV4 was completely re-written to use a more modular and flexible structure using events and actions. This replaced the 'fixed' actions that were available in CFv3.
The main coding differences are that CFv4 keeps all the form data in a $form->data array whereas CFV3 used the PHP $_POST and $_GET arrays. This means that the JRequest::set() methods used in CFV3 code no longer work in CFV4 - a $form->data variable needs to be set instead.
The other major difference is that the MooTools 1.4 library is used in Joomla! 2,5 and so JavaScript written for earlier versions will need to be updated to the current library.
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