respect-validation/library/Rules/PrimeNumber.php

<?php
namespace Respect\Validation\Rules;

class PrimeNumber extends AbstractRule
{
    public function validate($input)
    {
        if (is_numeric($input) && $input > 0) {
            $cont = 0;

            for ($i = 1; $i <= $input; $i++) {
                if (($input % $i) == 0) {
                    $cont = $cont + 1;
                }
            }

            if ($cont <= 2) {
                $input = 1;
            } else {
                $input = 0;
            }
        } else {
            $input = 0;
        }

        return (boolean) $input;
    }
}