What happens when a subclass overrides a property in PHP?

1-subclass-property-override.php

<?php

class Foo {
  protected $x = 'base'; // FIXME: Play with this line
  public function goFoo() {
    $this->x .= '-foo';
    return $this->x;
  }
}

class Bar extends Foo {
  //protected $x; // FIXME: Play with this line

  public function goBar() {
    $this->x .= '-bar';
    return $this->x;
  }
}

printf("\n=== PHP Version: %s\n", PHP_VERSION);

$a = new Bar();
print_r(array(
  'goFoo' => $a->goFoo()
));

print_r(array(
  'goBar' => $a->goBar()
));

print_r(array(
  'x' => $a->x,
));

2-observations.md

• The number of copies of $this->x in memory depends:
  • If parent declares private $x, then the parent and child store separate copies of $this->x.
  • If parent declares protected $x or public $x, then parent and child access the same copy of $this->x.
• The child class is allowed to loosen visibility but not tighten it.
  • Ex: If parent Foo has protected $x, then child Bar may declare public $x
  • Ex: If parent Foo has public $x, then child Bar may not declare protected $x.
• The child class changes the default value.
  • Ex: If parent Foo has protected $x = 'foo', and if child Bar has protected $x, then the default value of $x will be different. (Foo has default foo and Bar has default NULL`.)