If you make an ajax call similar to this :
http://example.com/ajax.php?firstParam=1
Within your ajax.php
file you can do something like this :
switch($_GET['firstParam']){
case "1":
callYourFunction();
break;
case "2":
someOtherFunction();
break;
}
This is a very stripped down example... In a real world case you would have to take security into account and sanitize any information you are getting from outside your server (i.e. from a user).
The names I have given are only place holders - you can change the function/variable names to whatever you feel comfortable with.
I hope this sheds some light on your conundrum.