Display error messages containing variables when validating forms in CakePHP

I have the $validate variable in my model which contains:

var $validate=array(

"username" => array( "usernameValid" => array( "rule" => "__alphaNumericDashUnderscore", "message" => "The username you entered is not valid!" ) ) );

The question is: how do I return an error message from the __alphaNumericDashUnderscore() validation method and throw it 开发者_运维百科in the message key in the rules array?

For example, this method, __alphaNumericDashUnderscore(), returns true or false, depending on the user input contains forbidden characters. But what if in this method, I would like to return which specific forbidden characters the user has typed and display them together with the messages? Something like "The username you entered is not valid! You have used the following forbidden characters: $chars".

Do you have any idea of how to achieve this? Thank you in advance.

---

By default, CakePHP validation method only return True or False. But it is still PHP. You can do anything. Here is my hack :

<?php
class User extends Model {
    var $name = 'User';
    var $invalidChars = "";
    var $validate=array("username" => array( "usernameValid" => array( 
        "rule" => "__alphaNumericDashUnderscore", 
        "message" => "The username you entered is not valid! You have used the following forbidden characters: $this->invalidChars" 
    )));

    function alphaNumericDashUnderscore($check) {
        // Process the value
        // Assign invalid char, $this->invalidChars = $chars
        // Return true or false
    }
}
?>