Please change require("file1.php"); to require("/absolute/path/to/file1.php"); and see if it works.

i have changed my code to absolute path.
require("F:/LXH/Webs/pware/file2.php");
$k= new B;
class B extends A{}

the problem is still there. a fatal error was thrown out.

require("F:/LXH/Webs/pware/file1.php");
a correction, it was file1.php, not file2.php
but im sure the problem is still there.

Expected behaviour.
Class "B" cannot be declared in compile time, because class "A" is declared in another file.
But in runtime, when you execute the line "$obj=new B;", the class "B" doesn't exist yet.

require("file1.php");
$j= new A;
$k= new B;
class B extends A{}

I've changed the code to ensure class A is defined.
as shown above, only the line where $k in throws error..

Please read what I said in my previous comment.

do you mean that it's forbidden to write code like
within one file:
class A{}
$j=new B;
class B extends A{}

or code like:

$j=new B;
class B{}

is also forbidden.

But at this moment they all work fine in php. if those mentioned are forbidden, maybe we should write something in the manual to clarify this...

No, I didn't mean this.
I meant exactly what I said - there is no way to declare this class in compile time, so it's declared in runtime.
And you get the error because you're trying to use the class which is not yet declared.
Please keep this report a bogus. Thank you.

All right. but you people have to notice this is still a design fault that should be addressed in somewhere. we php script developers don't know much about php's internal structure. we can hardly distinguish what is done in compile time and what is not. The problem isn't on the manual where you can learn. So people will be used to write code like this until something be pointed out or changed.