How can you get data from a url ID

Question

let us say we have a page with different IDs like id=1 with data of a, and id=2 with data of b

Answer 1

you should try:
var_dump($_GET);

Answer 2

$value_of_a = $_GET['a'];
same for b
you may also do a var_dump for the $_GET to see everything thay stored in the superglobal get