Access Level to certain class must be public error in PHP

I created this class

<?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));
        }
    }

for Validation purposes, but it kept giving me this error

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

But the instance variable in this class $_errors is declared public, I don't get it why I am receiving this error.

Youre getting that error because the visibility of the method must be the same or less restrictive than that of it its definition on a parent class. In this case you have addErrors as public on your abstract class and are attempting to make it protected on a child class.

As others have mentioned, you can't make a sub class method more restrictive than the parent; this is because sub classes are supposed to be a valid replacement for their parent class.

In your particular case, I would change the visibility of all methods and properties that start with an underscore to protected.

You've specify the protected access to the protected function _addErrors(array $errors) method of Form_Element_Validators class. So change it to public.

Edit:

Have you noticed? The sub class method (overridden method) is defined with Type Hinting. Please keep the same parameter type for both; super-class and sub-class method.

 abstract class Validator{
        public $_errors = array();
        abstract public function isValid($input);

        public function _addErrors(array $message){
            $this->_errors = $message;
        }
        ....

Since PHP 7.2.0 there was a bug #61970 fixed (Restraining __construct() access level in subclass gives a fatal error).

Bump your PHP version to 7.2 and you could make it private or protected.

It also happens when you are trying to do something with specific guard. You can add