What should I do if I want to allow only English alphabet to be validated?
Create your own rule?
var $validate = array(
'login' => array(
'rule' => '/^[a-z0-9]+$/i',
'message' => 'Only letters and integers'
));
http://book.cakephp.org/1.3/view/1179/Custom-Validation-Rules
See http://www.wiseguysonly.com/2009/11/27/a-workaround-for-the-cakephp-alphanumeric-issue/. The person in that article was having a different but related problem, and their solution is the same as the one you need.
The basic idea is to just use a custom regex. Change this:
'rule' => 'alphaNumeric'
To this:
'rule' => array('custom', '/^[a-z0-9]*$/i')
This should also work:
'rule' => '/^[a-z0-9]*$/i'
The official docs on custom regex validations are at http://book.cakephp.org/view/1179/Custom-Validation-Rules#Custom-Regular-Expression-Validation-1180.
Use a regex as the rule instead.