Class Extension, Object Oriented Programming in PHP

Continuing to unravel the Object Oriented Programming in PHP. This is an example of class extension method. The complete explanation is at http://us.php.net/manual/en/ref.classobj.php. Here is the code sample:

classes.inc

<?php

// base class with member properties and methods
class Vegetable {

  var $edible;
  var $color;

  function Vegetable($edible, $color="green")
  {
    $this->edible = $edible;
    $this->color = $color;
  }

  function is_edible()
  {
    return $this->edible;
  }

  function what_color()
  {
    return $this->color;
  }

} // end of class Vegetable

// extends the base class
class Spinach extends Vegetable {

  var $cooked = false;

  function Spinach()
  {
    $this->Vegetable(true, "green");
  }

  function cook_it()
  {
    $this->cooked = true;
  }

  function is_cooked()
  {
    return $this->cooked;
  }

} // end of class Spinach

?>

thepage.php

<?php

include "classes.inc";

// utility functions

function print_vars($obj)
{
  foreach (get_object_vars($obj) as $prop => $val) {
    echo "\t$prop = $val\n";
  }
}

function print_methods($obj)
{
  $arr = get_class_methods(get_class($obj));
  foreach ($arr as $method) {
    echo "\tfunction $method()\n";
  }
}

function class_parentage($obj, $class)
{
  if (is_subclass_of($GLOBALS[$obj], $class)) {
    echo "Object $obj belongs to class " . get_class($$obj);
    echo " a subclass of $class\n";
  } else {
    echo "Object $obj does not belong to a subclass of $class\n";
  }
}

// instantiate 2 objects

$veggie = new Vegetable(true, "blue");
$leafy = new Spinach();

// print out information about objects
echo "veggie: CLASS " . get_class($veggie) . "\n";
echo "leafy: CLASS " . get_class($leafy);
echo ", PARENT " . get_parent_class($leafy) . "\n";

// show veggie properties
echo "\nveggie: Properties\n";
print_vars($veggie);

// and leafy methods
echo "\nleafy: Methods\n";
print_methods($leafy);

echo "\nParentage:\n";
class_parentage("leafy", "Spinach");
class_parentage("leafy", "Vegetable");

?>

One important thing to note in the example above is that the object $leafy is an instance of the class Spinach which is a subclass of Vegetable, therefore the last part of the script above will output:

Parentage:
Object leafy does not belong to a subclass of Spinach
Object leafy belongs to class spinach a subclass of Vegetable