I have two files, let's say a.php and b.php:

a.php:

```
require 'b.php';
interface testInterface{}
class a implements testInterface{}

```

b.php:

```
class b extends a{}

```

When I run a.php, it throws this fatal error:

> 
> Fatal error: Class 'a' not found in b.php on line 2
> 
> 
> 

Two considerations that make it more interesting:

* Everything works fine if I remove the `implements` clause in the class `a`.
* Everything works fine if I put the class `b` in the same file as class `a`, without including it as an external file.

I can't understand why it's throwing that error, any help really appreciated.