Question

Imagine two classes which share almost the same exact methods and properties, both extending a parent class, but the differences are minimal.

class fields { 
 public function __construct() {
  global $id;
  $this->id = $id++;
 }
}

class input extends fields {
 public function __construct() {
  parent::__construct();
 }
 public function draw() {
  echo '<input>';
 }
}

class textarea extends fields {
 public function __construct() {
  parent::__construct();
 }
 public function draw() {
  echo '<textarea>';
 }
}

I’m thinking it would be more efficient to rewrite the textarea class in this psuedo-code fashion:

class textarea extends fields {
 public function __construct() {
  $this = new input(); // <<------ 
 }
 public function draw() {
  echo '<textarea>';
 }
}

Basically, I’m unsure how this would best be done so that the class acts like the class from the first example.

In essence, I would like to do the following using OOP, but be able to use the object as it can be in the first example above (be able to call the possibly overloaded methods, have different properties, etc.):

function a() {echo '123';}
function b() {a();}

I have just copied the entire class and modify a few lines, but I feel it is wasteful.

Final Answer
Thanks to those people, here is the combined answer with example calls:

abstract class fields { 
 private static $masterid = 0;
 public function __construct() {
  $this->id = self::$masterid++;
 }
}

class input extends fields {
 public $data;
 public function __construct($new = '') {
  parent::__construct();
  if ($new) $this->data = $new;
  else $this->data = 'Hello';
 }
 public function draw() {
  echo '<input>'.$this->export().'</input>';
 }
 public function export() {
  return 'ID '.$this->id.' = '.$this->data;
 }
}

class textarea extends input {
 public function __construct($new = '') {
  parent::__construct($new);
 }
 public function draw() {
  echo '<textarea>'.$this->export().'</textarea>';
 }
}

$a = new textarea();
$a->draw();
$a = new textarea('World');
$a->draw();
$a = new input('!');
$a->draw();

//Outputs:
// <textarea>ID 0 = Hello</textarea>
// <textarea>ID 1 = World</textarea>
// <input>ID 2 = !</input>

Answer 1

Make the fields class an abstract class, and like Darren suggested, make the ‘draw’ method a function of the fields class.

Now heres the trick, you want the input class to extend fields, but override the draw method. This will allow you to customize the functionality of that method, and you can still call the parent variation from within it.

Finally, since the textarea class is going to have many similarities to the input class, make textarea extend input. Thereby inheriting the properties and methods of both fields and input.