the use of the parent keyword when the inherited class is in a different file from the parent class produces an error:
(just by  writing both classes on a single file avoids the problem)

Fatal error: No parent class available in this context in file_of_inherited_class.php

example:

file: a.php
--------------
class a {
	function f() {
		echo "a::f()" ;
	}	
}

file: b.php
---------------
class b extends a {
	function f() {
		echo parent::f() ;	// <---- error when compiling 
		echo "b::f()" ;
	}	
}

$obj = new b() ;
$obj->f();