Validation d'un RIB en php

<?php
function isValidRib($rib)
{
    if(mb_strlen($rib) !== 23)
    {
        return false;
    }
    $key = substr($rib,-2);
    $bank = substr($rib,0,5);
    $bank = substr($rib,0,5);
    $branch = substr($rib,5,5);
    $account = substr($rib,10,11);
    $account = strtr($account,
    'ABCDEFGHIJKLMNOPQRSTUVWXYZ',
    '12345678912345678923456789');
    return 97 - bcmod(89*$bank + 15 * $branch + 3 * $account,97) === (int)$key;
}