Generating secure passwords in PHP

generatePassword.php

<?php
private function get_random_bytes($nb_bytes = 32)
{
    $bytes = openssl_random_pseudo_bytes($nb_bytes, $strong);
    if (false !== $bytes && true === $strong) {
        return $bytes;
    }
    else {
        throw new \Exception("Unable to generate secure token from OpenSSL.");
    }
}

// usage: $password = generate_password(12); // for a 12-char password containing [0-9, a-z, A-Z]
private function generate_password($length){
    return substr(
        preg_replace("/[^a-zA-Z0-9]/", "", base64_encode($this->get_random_bytes($length + 1))),
        0,
        $length
    );
}