How to make an ajax call using jquery to a PHP function that is inside a class?

What happens is that the declaration of a Class does not make it run, I mean you can have for example:

Class xxx {}

and then under the class you would have to create that object and call the function you need:

$c = new xxx();
echo $c->buscar($search);

This way when executing the result of the file is the printing of the result of the search function.

There are many more elegant ways to do this, but for your problem let's say that is enough.