Fix bad method names in README (getclass and get_type)

[Giorgiolino]

The getclass($data) call should be rewritten to get_class($data) (cf. https://www.php.net/get-class). The get_type($data) call should be rewritten to gettype($data) (cf. https://www.php.net/gettype)

Otherwise, it would result in the following Fatal Error (for getclass) PHP Fatal error: Uncaught Error: Call to undefined function getclass()