<?php

​

class Example {

    public const C_1 = 'val1';

    public const C_2 = 'val2';

    public const C_3 = 'val3';

​

    /**

    * @param string $c

    */

    public function __construct(string $c)

    {

       if (!in_array($c, [self::C_1, self::C_2], true)) {

            throw new InvalidArgumentException('Unsupported value');

        }

    }

}