SilverStripe Member signup page.

FixedConfirmedPasswordField.php

<?php

/**
 * Fix for issue https://github.com/silverstripe/silverstripe-framework/issues/2496
 */
class FixedConfirmedPasswordField extends ConfirmedPasswordField
{
	protected $useMemberValidator = false;

	public function getUseMemberValidator()
	{
		return $this->useMemberValidator;
	}

	public function setUseMemberValidator($value)
	{
		$this->useMemberValidator = $value == true;
		return $this;
	}

	public function setValue($value, $data = null) {
		// If $data is a DataObject, don't use the value, since it's a hashed value
		if ($data && $data instanceof DataObject) $value = '';

		//store this for later
		$oldValue = $this->value;

		if(is_array($value)) {
			$this->value = $value['_Password'];
			$this->confirmValue = $value['_ConfirmPassword'];

			if($this->showOnClick && isset($value['_PasswordFieldVisible'])) {
				$this->children->fieldByName($this->getName() . '[_PasswordFieldVisible]')
					->setValue($value['_PasswordFieldVisible']);
			}
		} else {
			if($value || (!$value && $this->canBeEmpty)) {
				$this->value = $value;
			}
		}

		//looking up field by name is expensive, so lets check it needs to change
		if ($oldValue != $this->value) {
			$this->children->fieldByName($this->getName() . '[_Password]')
				->setValue($this->value);

			$this->children->fieldByName($this->getName() . '[_ConfirmPassword]')
				->setValue($this->confirmValue);
		}

		return $this;
	}

	public function validate($validator)
	{
		// Use the member password validator for this field.
		// This way the validation happens on form-level and not when attempting to save the member to the DB.
		if($this->useMemberValidator){
			/** @var PasswordValidator $pwValidator */
			$pwValidator = Member::password_validator();
			if($pwValidator){
				/** @var ValidationResult $result */
				$result = $pwValidator->validate($this->value, null);

				if(!$result->valid()){
					$validator->validationError(
						$this->name,
						$result->message(),
						"validation",
						false
					);

					return false;
				}
			}
		}

		return parent::validate($validator);
	}
}

SignupPage.php

<?php

class SignupPage extends Page
{
	private static $description = 'Member signup page.';
}

class SignupPage_Controller extends Page_Controller
{
	private static $allowed_actions = array(
		'AccountRegisterForm'
	);

	public function AccountRegisterForm()
	{
		$fields = FieldList::create(array(
			TextField::create('FirstName', _t('Member.FIRSTNAME', 'Firstname')),
			TextField::create('Surname', _t('Member.SURNAME', 'Surname')),
			EmailField::create('Email', _t('Member.EMAIL', 'Email')),
			// Use a fixed password field because of https://github.com/silverstripe/silverstripe-framework/issues/2496
			$pwField = FixedConfirmedPasswordField::create('Password', _t('Member.PASSWORD', 'Password'))
		));

		$pwField->setUseMemberValidator(true)->setCanBeEmpty(false);

		$validator = Member_Validator::create(array('FirstName', 'Surname', 'Email', 'Password'));

		$actions = FieldList::create(array(
			FormAction::create('doCreateAccount', _t('SignupPage.CREATE_ACCOUNT', 'Create'))
		));

		$form = Form::create($this, 'AccountRegisterForm', $fields, $actions, $validator);
		return $form;
	}

	public function doCreateAccount($data, $form)
	{
		$submittedData = $data;
		if(is_array($submittedData['Password'])){
			// confirmed password field stores the field value in an array which will cause
			// errors upon creating the new Member.
			$submittedData['Password'] = $data['Password']['_Password'];
		}

		/** @var Member $member */
		$member = Member::create($submittedData);
		try {
			$member->write();

			if($group = SiteConfig::current_site_config()->CustomerGroup()){
				$member->Groups()->add($group);
			}

			$member->logIn();
			if($accountPage = $this->AccountPage()){
				return $this->redirect($accountPage->AbsoluteLink());
			}

			$form->sessionMessage(_t('SignupPage.USER_CREATED', 'The account was successfully created'), 'good');
		} catch(Exception $e){
			$form->sessionMessage($e->getMessage(), 'bad');
		}
		return $this->redirectBack();
	}
	
	public function AccountPage()
	{
		return AccountPage::get()->first();
	}
}