PHP Property Visibilty

05 Apr 2013 Joshua Chamberlain

It seems like parents just can’t keep track of their children these days! A while back I wrote a little code that seemed fine to me but returned a surprising fatal error. Of course I knew that private properties and methods are visible only to the class that defined them, but one particular implication of that fact had not yet crossed my mind. Consider the following bits of code:

<?php // ParentClass.php
class ParentClass {
  private $privateVar = 'private';
  protected $protectedVar = 'protected';
  public $publicVar = 'public';

  public function testMethod() {
    echo '<pre>';
    echo $this->privateVar."\n";
    echo $this->protectedVar."\n";
    echo $this->publicVar."\n";
    echo '</pre>';

<?php // index.php
include 'ParentClass.php';

$test = new ParentClass();

When index.php is loaded, we’ll see all three properties displayed just as expected.


Let’s extend the parent class:

<?php // ChildClass.php
class ChildClass extends ParentClass {
  private $privateVar = 'private child';
  protected $protectedVar = 'protected child';
  public $publicVar = 'public child';
<?php // index.php
include 'ParentClass.php';
include 'ChildClass.php';

$test = new ChildClass();

Having overridden all three properties of the parent class, loading index.php yields a different but expected (for me) result.

protected child
public child

The original testMethod() is defined in ParentClass and thus has access only to the original $privateVar, not the new version in ChildClass. The protected and public properties are overridden and thus show their new values.

Here’s where I ran into trouble. Consider the following method, simplified from the controller class of an old framework I built a while back:

// ...
protected function loadView($name, $data=array()) {
  $file = APATH.'views/'.$name.'.php';
  if(file_exists($file)) {
    $html = ob_get_contents()."\n";
    $this->html .= $html;
    return true;
  return false;

As you can see, it’s including a view file, giving it access to certain data, and then saving the output. Since the file is included within a class method, whatever’s in the file has access to the class properties and methods just like any other part of the class, so sometimes for convenience I would use $this within the view file (probably a bad idea), such as when grabbing $this->user. Once I tried accessing a private property from the view and was surprised by the accompanying fatal error. It went something like this:





class Page extends Controller {
  private $pageID = 1;

  public function index() {
    // ...
    $this->loadView('view.php', array());

The loadView() method was defined in the Controller class, but run in the Page class where the private $pageID was declared. There was no $pageID in the Controller class, but instead of just echoing NULL, PHP recognized my code as an illegal attempt to access a private property in Page from a method in Controller and returned a fatal error. And that’s what really confused me. Instead of acting like the property didn’t exist in Controller (it didn’t), the interpreter correctly ascertained what I wanted to do and refused to do it.

So apparently the PHP interpreter is smarter than I am.