生成数字字母验证码

/**
 * @param string $str 生成验证码的字符
 * @param int $len
 * @param string $format
 * @param string $salt
 * @return bool|string
 */
function genCode($str, $len = 4, $format = 'NUMBER', $salt = '')
{
    $salt = $salt ? $salt : 'secret string';
    if (!$str) {
        return false;
    }
    $_str = $str . $salt;
    $_str = trim(base64_encode(md5($_str)), '=');
    switch ($format) {
        case 'ALL':
            $chars = preg_replace('/0|o|O/', '', $_str);
            break;
        case 'CHAR':
            $chars = preg_replace('/\d|o|O/', '', $_str);
            break;
        case 'NUMBER':
        default:
            $chars = base_convert($_str, 36, 10);
            $chars = str_replace('0', '', $chars);
            break;
    }
    return substr($chars, 0, $len);
}