ZF-7727: Format of the message key not the same for a missing field in Zend_Filter_Input
When a field is missing, the messages returned as an array is of the following form (with '0' as the key of the message) :
Array ( [DATE_DAY] => Array (  => Field 'DATE_DAY' is required by rule 'DATE_DAY', but the field is missing ) )
Whereas the list of messages for an empty field or any other validation errors will look like (with the name of the validation rule as the key of each message) :
[Array ( [DATE_DAY] => Array ( [isEmpty] => You must give a non-empty value for field 'DATE_DAY' ) ) Array ( [DATE_DAY] => Array ( [notDigits] => 'a' contains not only digit characters [notBetween] => 'a' is not between '1' and '31', inclusively ) )
So I guess the key of the message for a missing field should follow the same pattern and be something like 'isRequired'.