[RESOLVED] Inheritance question

[potatoCode]

Hello php gurus!

I'm trying to make a simple template generator that's going to be used with XML files.
In doing so, I've laid out a simple structure, but before I can even get to work on the template generator,
I've run into some problems with inheritance problems.
Basic structure is

interface c {}

abstract class a {}

class b extends a implements c {}

$d = new b();

which works fine.
But the same structure with my code doesn't work.
Here's the simplified version of my code

interface class

<?php
interface Core
{
    function register($obj);
    function unregister($obj);
}
?>

abstract class

<?php
abstract class Engine
{
    // ?return [boolean]
    abstract protected function hireEngineer();

// ?return [boolean]
abstract protected function fireEngineer();

// ?return [int]
protected function attach($part, $type)
{
    if(DEBUG)
        printf("#DEBUG# attaching engine parts of ".var_dump($refPart));

    if(!is_object($part))
        exit();

    $this->coreID = $type."_".get_class($part);

    if(!$this->registry[$this->coreID] = $part)
        exit();

    $this->clearID();
    return count($this->registry);
}

// ?return [boolean]
protected function detach($part, $type)
{
    $this->coreID = $type."_".get_class($part);
    if(DEBUG)
        printf("#DEBUG# detaching engine parts of ".var_dump($part));

    if($key = array_search($this->coreID, $this->registry))
    {
        unset($this->registry[$key]);            
    }

    $this->clearID();
    return true;
}

// ?return [void]
private function clearID()
{
    $this->coreID = "";
}

///////////////////////////////////////////////////////////////////
protected $registry;
protected $coreID;
}
?>

And the class that inherits the abstract class

<?php
final class Input extends Engine implements Core
{
    public function __construct()
    {
        parent::$registry = array();
        parent::$coreID = null;
    }

// ?return [boolean]
public function hireEngineer()
{
    ; // do nothing for now
}

// ?return [boolean]
public function fireEngineer()
{
    ; // do nothing for now
}

// ?return [int]
public function register($obj)
{
    return $this->attach($obj, "Input");
}

public function unregister($obj)
{
    return $this->detach($obj, "Input");
}
}
?>

I get the "Cannot instantiate an abstract class Input".
I just can't figure out why the compiler thinks Input is an abstract class.

I'd appreciate if anyone could help.

EDIT:
What is the order of object creation in inheritance in php?
I can't help but feeling this error is somewhat similar to passing this to the base class in C++...

EDIT: fixed some typos

[bpat1434]

The only thing I can see is that ultimately in the constructor it should look like this:

    public function __construct()
    {
        $this->registry = array();
        $this->coreID = null;
    } 

since it's parent isn't really an instantiated object.

Of course if registry and coreID will always be an array and null (or integer) respectively, you could define that in the abstract class just as well:

protected $registry = array();
protected $coreID = null;

Then you don't have to worry about initializing them in the constructor of sub-classes (unless you want to).

[johanafm]

It looks fine to me, except that you have

abstract class Engine {
	protected $coreID;	// not static
	protected static staticCoreID;	// since this is static it can be referenced with parent:: etc
}

final class Input {
	self::$coreID;	// Error
	$this->coreID;	// ok
	self::$staticCoreID;		// ok
	parent::$staticCoreID		// ok, and means same as above
	static::$staticCoreID		// ok, and means same as above

// If you however
protected static $staticCoreID = "some other value";
// Now self:: and static:: would mean something other than parent::
}
}

[potatoCode]

Thank you bpat1434 and johanafm.
I am really learning a great deal here from here.

As for the problem,
there was another file with the same class name and of course, it was declared as abstract.
It was one of the skeletal pre-design files I forgot to delete afterwards.

It took me painful hours to figure it out but the problem is now solved.
Thanks to two you once again.