Generate a random string in PHP 5.6 using mt_rand

Strings.php

<?php

class Strings
{
    /**
     * Generate a secure random string
     *
     * @param $length
     * @param string $characters
     * @param string $encoding
     * @throws \BadMethodCallException
     * @return string
     */
    public static function random(
        $length = 32,
        $characters = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ',
        $encoding = '8bit'
    ) {
        if (0 === $length) {
            throw new \BadMethodCallException('Please enter a length bigger than 0');
        }
        if ('' === $characters) {
            throw new \BadMethodCallException('Please enter some characters');
        }
        if (false === ($max = mb_strlen($characters, $encoding))) {
            throw new \BadMethodCallException('Invalid encoding passed');
        }
        $string = '';
        $max--;
        for ($i = 0; $i < $length; ++$i) {
            $string .= $characters[mt_rand(0, $max)];
        }
        return $string;
    }
}