Ok...the docs are a bit vague when it comes to an abstract class extending another abstract class.  An abstract class that extends another abstract class doesn't need to define the abstract methods from the parent class.  In other words, this causes an error:

<?php
abstract class class1 {
  abstract public function someFunc();
}
abstract class class2 extends class1 {
  abstract public function someFunc();
}
?>

Error: Fatal error: Can't inherit abstract function class1::someFunc() (previously declared abstract in class2) in /home/sneakyimp/public/chump.php on line 7

However this does not:

<?php
abstract class class1 {
  abstract public function someFunc();
}
abstract class class2 extends class1 {
}
?>

An abstract class that extends an abstract class can pass the buck to its child classes when it comes to implementing the abstract methods of its parent abstract class.