switch(true)
{
    case (strlen($foo) > 30):
        $error = "The value provided is too long.";
    $valid = false;
    break;

    case (!preg_match(‘/^[A-Z0-9]+$/i‘, $foo)):
        $error = "The value must be alphanumeric.";
    $valid = false;
    break;

    default:
    $valid = true;
    break;
}