Programmatically: How to use Magento's custom validation mechanism for custom form validation

Step 1: Create a custom validation class

Create a new PHP file, e.g., MyCompany/MyModule/Model/Form/Validator/MyValidation.php:

	class MyCompany_MyModule_Model_Form_Validator_MyValidation
    extends Mage_Core_Model_Form_Validator_Abstract
  {
    public function validate($object)
    {
        // Your custom validation logic goes here
        // For example, let's say we want to validate a password field
        $password = $object->getData('password');
        if (strlen($password) < 8) {
            $this->_addError('Password must be at least 8 characters');
        }
        return $this->_getMessages();
    }
 }
	

Step 2: Register the validation class in your module's config file

Add the following code to your module's etc/config.xml file:

	<?xml version="1.0"?>
	<config>
    <!-- ... -->
    <global>
        <models>
            <mycompany_mymodule>
                <class>MyCompany_MyModule_Model</class>
            </mycompany_mymodule>
        </models>
        <forms>
            <myform_validator>
                <class>MyCompany_MyModule_Model_Form_Validator_MyValidation</class>
            </myform_validator>
        </forms>
    </global>
    <!-- ... -->
	</config>
	

Step 3: Use the custom validation class in your form

Modify your form's XML file (e.g., app/design/frontend/yourtheme/template/form.phtml) to include the custom validation class:

	<?xml version="1.0"?>
	<form xmlns="urn:magento:frontend:en_us">
		<!-- ... -->
		<field name="password" validate="myform_validator">
			<!-- ... -->
		</field>
		<!-- ... -->
	</form>
	

Step 4: Test your custom validation

Submit your form with a password that is too short (e.g., less than 8 characters). You should see an error message indicating that the password is too short.