I have a class (myclass) that extends an abstract class (abby) and getting a fatal error saying the abstract class is not found when including the classes.

abby.php
<?
abstract class abby {
}
?>

myclass.php
<?
class myclass extends abby {
}
?>

main.php
<?
include 'abby.php';
include 'myclass.php';
$obj=new myclass();
?>

When I run main.php I am getting a fatal error:
PHP Fatal error:  Class 'abby' not found

Note: if I include the abstract class in my class file then I do NOT get the error.
myclass.php
<?
include 'abby.php';
class myclass extends abby {
}
?>