如何在CodeIgniter中扩展多个类？

How can I extend multiple classes like this:

class Backend_api extends CI_Controller, Appointments
{
        do stuff...
}

As mentioned, multiple inheritance is not allowed in php, but if you need to access functions in the Appointments class, why not just create an instance of it in the class and call the function on that instance?

$ins = new Appointments();
$result = $ins->someFunction();

PHP does not support multiple inheritance

As already mentioned, PHP does not support multiple inheritance. But since PHP5.4 you can use traits and use them in your classes. More info about traits here