C

static void zephir_get_class_ns(zval *result, zval *object, int lower)
{
	int found = 0;
	unsigned int i, class_length;
	char *cursor, *class_name;
	zval *z = Z_ISREF_P(object) ? Z_REFVAL_P(object) : object;

	if (Z_TYPE_P(z) != IS_OBJECT) {
		if (Z_TYPE_P(z) != IS_STRING) {
			ZVAL_NULL(result);
			php_error_docref(NULL, E_WARNING, "zephir_get_class_ns expects an object");
			return;
		}

		class_name   = Z_STRVAL_P(z);
		class_length = Z_STRLEN_P(z);
	}
	else {
		zend_class_entry *ce = Z_OBJCE_P(z);
		class_name   = ZSTR_VAL(ce->name);
		class_length = ZSTR_LEN(ce->name);
	}

	if (!class_length) {
		ZVAL_NULL(result);
		return;
	}

	i = class_length;
	cursor = (char *) (class_name + class_length - 1);

	while (i > 0) {
		if ((*cursor) == '\\') {
			found = 1;
			break;
		}
		cursor--;
		i--;
	}

	if (found) {
		int cursor_length = class_length - i;
		cursor = (char *) emalloc(cursor_length + 1);
		memcpy(cursor, class_name + i, cursor_length);
		cursor[cursor_length] = 0;
		ZVAL_STRING(result, cursor);
		efree(cursor);
	} else {
		ZVAL_STRINGL(result, class_name, class_length);
	}

	if (lower) {
		zend_str_tolower(Z_STRVAL_P(result), Z_STRLEN_P(result));
	}

}

PHP

function zephir_get_class_ns($object, int $lower): ?string
{
    if (!is_object($object)) {
        if (!is_string($object)) {
            throw \Phalcon\Error('zephir_get_class_ns expects an object');
        }

        $className = $object;
        $classLength = strlen($object);
    } else {
        $className = get_class($object);
        $classLength = strlen($className);
    }

    if (!$classLength) {
        return null;
    }

    $cursor = $classLength - 1;
    $i = $classLength;

    $found = 0;
    while ($i > 0) {
        if ($className[$cursor] == '\\') {
            $found = 1;
            break;
        }

        $cursor--;
        $i--;
    }

    if ($found) {
        $result = substr($className, 0, $cursor);
    } else {
        $result = $className;
    }

    if ($lower) {
        $result = strtolower($className);
    }

    return $result;
}

Example:

$class = 'App\Model\User';

echo zephir_get_class_ns($class, 0); // 'App\Model'