Interpreter::execute PHP Method

    function execute($string)
    {
        $expression = null;
        for ($i = 0; $i < strlen($string); $i++) {
            $temp = $string[$i];
            switch (true) {
                case is_numeric($temp):
                    $expression = new ExpressionNum();
                    break;
                default:
                    $expression = new ExpressionCharater();
            }
            echo $expression->interpreter($temp);
            echo "<br>";
        }
    }

        }
    }
}
class ExpressionCharater extends Expression
{
    public function interpreter($str)
    {
        return strtoupper($str);
    }
}
class Interpreter
{
    public function execute($string)
    {
        $expression = null;
        for ($i = 0; $i < strlen($string); $i++) {
            $temp = $string[$i];
            switch (true) {
                case is_numeric($temp):
                    $expression = new ExpressionNum();
                    break;
                default:
                    $expression = new ExpressionCharater();
            }
            echo $expression->interpreter($temp);
        }
    }
}
$obj = new Interpreter();
$obj->execute("12345abc");