gen_random_string function will never find 36th char

strlen($characters) would return 36 but php index starts from 0 and not 1. Which means you’d never have $characters[36] which will return a php notice.

Here is an updated function:

private function gen_random_string($len) {
        $length = intval($len);
        $characters = '0123456789abcdefghijklmnopqrstuvwxyz';
        $string = '';
        for ($p = 0; $p < $length; $p++) {
            $string .= $characters[mt_rand(0, strlen($characters)-1)];
        }
        return $string;
    }