对某些类的访问级别必须是 PHP 中的公共错误

我创建了这个类

<?php
    abstract class Validator{
        public $_errors = array();
        abstract public function isValid($input);
        public function _addErrors($message){
            $this->_errors = $message;
        }
        public function getErrors(){
            return $this->_errors;
        }

        public function getMessage(){
            return $this->message;
        }
    }
    class Validator_NoSpaces extends Validator{
        public function __construct($value){
            $this->isValid($value);
        }
        public function isValid($value){
                if (preg_match('/'s/', $value)){
                $this->_addErrors("Spaces are not allowed");
                return false;
            }
            return true;
        }       
    }
    class Validator_MinimumLength extends Validator{
        protected $_minLength;
        protected $value;
        public function __construct($value ,$minLength=8){
            $this->_minLength = $minLength;
            $this->value = $value;
            $this->isValid($value);
        }
        public function isValid($input){
             if (strlen($input) > $this->_minLength) {
                 return true;
            }else{
                $this->_addErrors("Input must be at least {$this_minLength}");
                return false;
          }
        }
    }
    class Form_Element_Validators extends Validator{
        protected $_validators = array();
    public function addValidator(Validator $validator)
    {
        $this->_validators[] = $validator;
    }
    public function getValidators()
    {
        return $this->_validators;
    }
    protected function _addErrors(array $errors)
    {
        foreach ($errors as $error) {
            $this->_addErrors($error);
        }
    }
    public function hasErrors()
    {
        return (count($this->getErrors()) !== 0);
    }
    public function isValid($input)
    {
        foreach ($this->_validators as $validator) {
            if (!$validator->isValid($input)) {
                $this->_addErrors($validator->getErrors());
            }
        }
        return !$this->hasErrors();
    }
    }
    class Form_Element extends  Form_Element_Validators{
        public function __construct($value){
              $this->addValidator(new Validator_NoSpaces($value));
              $this->addValidator(new Validator_MinimumLength($value));
        }
    }

出于验证目的，但它一直给我这个错误

Fatal error: Access level to Form_Element_Validators::_addErrors() must be public (as in class Validator) in C:'xampp'htdocs'beatbeast'includes'Db'Validators.php on line 91

但是这个类 $_errors 中的实例变量被声明为公共的，我不明白为什么我会收到此错误。