gistfile1.php

abstract class Entity {
    /* snip */
    public function bindValidation($field, callable $validator) {
        $this->validationFunctions[$field] = $validator;
    }
    
    public function validate() {
        foreach ($this->validationFunction as $field => $validator) {
            if ($validator($this->getField($field)) === false) {
                return false;
            }
        }
        return true;
    }
}

gistfile2.php

class SomeService {
    public function createUser($username, $password) {
        $user = $this->entityFactory->create("user");
        $user->setUserame($username);
        $user->bindValidation("username", function($username) { return !is_empty($username); });
        $user->setPassword($password);
        $user->bindValidation("password", function($password) { return (!is_empty($password) && strlen($password) > 6); });
        
        if (!$user->validate()) {
            throw new ValidationException("...");
        }
        
        $mapper = $this->mapperFactory->create("user");
        $mapper->map($user);
    }
}