Zend_Validate supplies a set of commonly needed validators, but inevitably, developers will wish to write custom validators for their particular needs. The task of writing a custom validator is facilitated by implementing Zend_Validate_Interface.

Zend_Validate_Interface defines two methods, isValid() and getMessages(), that may be implemented by user classes. An object that implements this interface may be added to a validator chain with Zend_Validate::addValidator().

The following example demonstrates how a custom validator might be written:

<?php

require_once 'Zend/Validate/Interface.php';

class MyValidator implements Zend_Validate_Interface
{
    protected $_messages = array();

    public function isValid($value)
    {
        $this->_messages = array();

        // arrive on $requirement based on validation criteria

        if (!$requirement) {
            $this->_messages[] = "Reason validation failed";
            return false;
        }

        return true;
    }

    public function getMessages()
    {
        return $this->_messages;
    }
}

?>

To add an instance of the above validator to a validator chain:

<?php

$validatorChain = new Zend_Validate();
$validatorChain->addValidator(new MyValidator());

?>