Expected

<?php
abstract class AbstractValidator
{
    private $_args;

    protected const array ALLOWED_VALUES = [];

    public function __construct(array $args)
    {
        $this->_args = $args;
    }

    public function isValid(): bool
    {
        foreach ($this->_args as $value)
            if (!in_array(static::ALLOWED_VALUES, $value))
                return false;
        return true;
    }
}

class NamesValidator extends AbstractValidator
{
    //type could be omitted since it has to follow the parents signature
    protected const ALLOWED_VALUES = [
        'Mark', 'Bill', 'Dan'
    ];
}

class InvalidNamesValidator extends AbstractValidator
{
    //this would throw a TypeError
    protected const ALLOWED_VALUES = 0;
}